Add default --empty to handle empty values (#15223)

# Description

Adds a new `--empty/-e` flag to the `default` command.

# User-Facing Changes

Before:

```nushell
$env.FOO = ""
$env.FOO = $env.FOO? | default bar
$env.FOO
# => Empty string
```

After:

```nushell
$env.FOO = ""
$env.FOO = $env.FOO? | default -e bar
$env.FOO
# => bar
```

* Uses `val.is_empty`, which means that empty lists and records are also
replaced
* Empty values in tables (with a column specifier) are also replaced.

# Tests + Formatting

7 tests added and 1 updated + 1 new example

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

N/A

crates/nu-command/tests/commands/default.rs

@@ -5,10 +5,13 @@ fn adds_row_data_if_column_missing() {
     let sample = r#"
                 {
                     "amigos": [
-                        {"name":   "Yehuda"},
+                        {"name": "Yehuda"},
                         {"name": "JT", "rusty_luck": 0},
-                        {"name":   "Andres", "rusty_luck": 0},
-                        {"name":"GorbyPuff"}
+                        {"name": "Andres", "rusty_luck": 0},
+                        {"name": "Michael", "rusty_luck": []},
+                        {"name": "Darren", "rusty_luck": {}},
+                        {"name": "Stefan", "rusty_luck": ""},
+                        {"name": "GorbyPuff"}
                     ]
                 }
             "#;
@@ -44,3 +47,68 @@ fn replaces_null() {
     let actual = nu!(r#"null | default 1"#);
     assert_eq!(actual.out, "1");
 }
+
+#[test]
+fn adds_row_data_if_column_missing_or_empty() {
+    let sample = r#"
+                {
+                    "amigos": [
+                        {"name": "Yehuda"},
+                        {"name": "JT", "rusty_luck": 0},
+                        {"name": "Andres", "rusty_luck": 0},
+                        {"name": "Michael", "rusty_luck": []},
+                        {"name": "Darren", "rusty_luck": {}},
+                        {"name": "Stefan", "rusty_luck": ""},
+                        {"name": "GorbyPuff"}
+                    ]
+                }
+            "#;
+
+    let actual = nu!(pipeline(&format!(
+        "
+                {sample}
+                | get amigos
+                | default -e 1 rusty_luck
+                | where rusty_luck == 1
+                | length
+            "
+    )));
+
+    assert_eq!(actual.out, "5");
+}
+
+#[test]
+fn replace_empty_string() {
+    let actual = nu!(r#"'' | default -e foo"#);
+    assert_eq!(actual.out, "foo");
+}
+
+#[test]
+fn do_not_replace_empty_string() {
+    let actual = nu!(r#"'' | default 1"#);
+    assert_eq!(actual.out, "");
+}
+
+#[test]
+fn replace_empty_list() {
+    let actual = nu!(r#"[] | default -e foo"#);
+    assert_eq!(actual.out, "foo");
+}
+
+#[test]
+fn do_not_replace_empty_list() {
+    let actual = nu!(r#"[] | default 1 | length"#);
+    assert_eq!(actual.out, "0");
+}
+
+#[test]
+fn replace_empty_record() {
+    let actual = nu!(r#"{} | default -e foo"#);
+    assert_eq!(actual.out, "foo");
+}
+
+#[test]
+fn do_not_replace_empty_record() {
+    let actual = nu!(r#"{} | default {a:5} | columns | length"#);
+    assert_eq!(actual.out, "0");
+}