Skip decoration lines for detect columns --guess (#13274)

# Description
I introduced a regression in #13272 that resulted in `detect columns
--guess` to panic whenever it had to handle empty, whitespace-only, or
non-whitespace-only lines that go all the way to the last column (and as
such, cannot be considered to be lines that only have entries for the
first colum). I fix this by detecting these cases and skipping them,
since these are usually decoration lines. An example is the second line
output by `winget list`:

![image](https://github.com/nushell/nushell/assets/20356389/06c873fb-0a26-45dd-b020-3bcc737d027f)

What we don't want to skip, however, is lines that contain no
whitespace, and fit into the detected first column, since these lines
represent cases where data is only available for the first column, and
are not just decoration lines. For example (made up example, there are
no such entries in `winget lits`'s output), in this output we would not
want to skip the `Docker Desktop` line :
```
Name                                                        Id                                           Version     Available Source
-------------------------------------------------------------------------------------------------------------------------------------
AMD Software                                                ARPMachineX64AMD Catalyst Install Manager 24.4.1
AMD Ryzen Master                                            ARPMachineX64AMD Ryzen Master             2.13.0.2908
Docker Desktop
Mozilla Firefox (x64 en-US)                                 Mozilla.Firefox                              127.0.2               winget
```

![image](https://github.com/nushell/nushell/assets/20356389/12e31995-a7c1-4759-8c62-fb4fb199fd2e)

NOTE: `winget list | detect columns --guess` does not panic, but sadly
still does not work as expected. I believe this is not a nushell issue
anymore, but a `winget` one. When being piped, `winget` seems to add
extra whitespace and random `\r` symbols at the beginning of the text.
This messes with the column detection, of course.

![image](https://github.com/nushell/nushell/assets/20356389/7d1b7e5f-17d0-41c8-8d2f-7896e0d73d66)

![image](https://github.com/nushell/nushell/assets/20356389/56917954-1231-43e7-bacf-e5760e263054)

![image](https://github.com/nushell/nushell/assets/20356389/630bcfc9-eb78-4a45-9c8f-97efc0c224f4)


# User-Facing Changes
`detect columns --guess` should not panic when receiving output from
`winget list` at all anymore.

A breaking change is the skipping of decoration lines, especially since
scripts probably were doing something like
`winget list | lines | reject 1 | str join "\n" | detect columns
--guess`. This will now cause them to reject a line with valid data.

# Tests + Formatting
Added tests that exercise these edge cases, as well as a single-column
test to make sure that trivial cases keep working.

# After Submitting
<!-- If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.
-->

crates/nu-command/src/strings/guess_width.rs

@@ -72,7 +72,9 @@ impl GuessWidth {
 
         let mut rows = Vec::new();
         while let Ok(columns) = self.read() {
-            rows.push(columns);
+            if !columns.is_empty() {
+                rows.push(columns);
+            }
         }
         rows
     }
@@ -180,6 +182,19 @@ fn split(line: &str, pos: &[usize], trim_space: bool) -> Vec<String> {
     let (line_char_boundaries, line_chars): (Vec<usize>, Vec<char>) = line.char_indices().unzip();
     let mut w = 0;
 
+    if line_chars.is_empty() || line_chars.iter().all(|&c| c.is_whitespace()) {
+        // current line is completely empty, or only filled with whitespace
+        return Vec::new();
+    } else if !pos.is_empty()
+        && line_chars.iter().all(|&c| !c.is_whitespace())
+        && pos[0] < UnicodeWidthStr::width(line)
+    {
+        // we have more than 1 column in the input, but the current line has no whitespace,
+        // and it is longer than the first detected column separation position
+        // this indicates some kind of decoration line. let's skip it
+        return Vec::new();
+    }
+
     for p in 0..line_char_boundaries.len() {
         if pos.is_empty() || n > pos.len() - 1 {
             start_char = p;
@@ -463,6 +478,122 @@ Ștefan         Țincu ";
         assert_eq!(got, want);
     }
 
+    #[test]
+    fn test_guess_width_single_column() {
+        let input = "A
+
+B
+
+C";
+
+        let r = Box::new(std::io::BufReader::new(input.as_bytes())) as Box<dyn std::io::Read>;
+        let reader = std::io::BufReader::new(r);
+
+        let mut guess_width = GuessWidth {
+            reader,
+            pos: Vec::new(),
+            pre_lines: Vec::new(),
+            pre_count: 0,
+            limit_split: 0,
+        };
+
+        let want = vec![vec!["A"], vec!["B"], vec!["C"]];
+        let got = guess_width.read_all();
+        assert_eq!(got, want);
+    }
+
+    #[test]
+    fn test_guess_width_row_without_whitespace() {
+        let input = "A B C D
+-------
+E F G H";
+
+        let r = Box::new(std::io::BufReader::new(input.as_bytes())) as Box<dyn std::io::Read>;
+        let reader = std::io::BufReader::new(r);
+
+        let mut guess_width = GuessWidth {
+            reader,
+            pos: Vec::new(),
+            pre_lines: Vec::new(),
+            pre_count: 0,
+            limit_split: 0,
+        };
+
+        let want = vec![vec!["A", "B", "C", "D"], vec!["E", "F", "G", "H"]];
+        let got = guess_width.read_all();
+        assert_eq!(got, want);
+    }
+
+    #[test]
+    fn test_guess_width_row_with_single_column() {
+        let input = "A B C D
+E
+F G H I";
+
+        let r = Box::new(std::io::BufReader::new(input.as_bytes())) as Box<dyn std::io::Read>;
+        let reader = std::io::BufReader::new(r);
+
+        let mut guess_width = GuessWidth {
+            reader,
+            pos: Vec::new(),
+            pre_lines: Vec::new(),
+            pre_count: 0,
+            limit_split: 0,
+        };
+
+        let want = vec![
+            vec!["A", "B", "C", "D"],
+            vec!["E"],
+            vec!["F", "G", "H", "I"],
+        ];
+        let got = guess_width.read_all();
+        assert_eq!(got, want);
+    }
+
+    #[test]
+    fn test_guess_width_empty_row() {
+        let input = "A B C D
+
+E F G H";
+
+        let r = Box::new(std::io::BufReader::new(input.as_bytes())) as Box<dyn std::io::Read>;
+        let reader = std::io::BufReader::new(r);
+
+        let mut guess_width = GuessWidth {
+            reader,
+            pos: Vec::new(),
+            pre_lines: Vec::new(),
+            pre_count: 0,
+            limit_split: 0,
+        };
+
+        let want = vec![vec!["A", "B", "C", "D"], vec!["E", "F", "G", "H"]];
+        let got = guess_width.read_all();
+        assert_eq!(got, want);
+    }
+
+    #[test]
+    fn test_guess_width_row_with_only_whitespace() {
+        let input = "A B C D
+       
+E F G H";
+
+        let r = Box::new(std::io::BufReader::new(input.as_bytes())) as Box<dyn std::io::Read>;
+        let reader = std::io::BufReader::new(r);
+
+        let mut guess_width = GuessWidth {
+            reader,
+            pos: Vec::new(),
+            pre_lines: Vec::new(),
+            pre_count: 0,
+            limit_split: 0,
+        };
+
+        let want = vec![vec!["A", "B", "C", "D"], vec!["E", "F", "G", "H"]];
+        let got = guess_width.read_all();
+        assert_eq!(got, want);
+    }
+
     #[test]
     fn test_to_table() {
         let lines = vec![