upserting data of a cellpath that doesn't exist into a record creates the cellpath (#9257)

# Description
Fixes #9254.

# User-Facing Changes
upserting data of a cellpath that doesn't exist into a record now
creates the cellpath.

# Tests + Formatting

```
~/CodingProjects/nushell> mut a = {}                                                                       
~/CodingProjects/nushell> $a.b.c = 99                                                                            
~/CodingProjects/nushell> $a                                                                                    
╭───┬────────────╮
│   │ ╭───┬────╮ │
│ b │ │ c │ 99 │ │
│   │ ╰───┴────╯ │
╰───┴────────────╯
```

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

@@ -171,3 +171,9 @@ fn mut_path_operator_assign() {
 
     assert_eq!(actual.out, "5");
 }
+
+#[test]
+fn mut_records_update_properly() {
+    let actual = nu!(pipeline("mut a = {}; $a.b.c = 100; $a.b.c"));
+    assert_eq!(actual.out, "100");
+}

crates/nu-protocol/src/value/mod.rs

@@ -1036,8 +1036,8 @@ impl Value {
                             }
                         }
                         if !found {
+                            cols.push(col_name.clone());
                             if cell_path.len() == 1 {
-                                cols.push(col_name.clone());
                                 vals.push(new_val);
                             } else {
                                 let mut new_col = Value::Record {