Fix signature of split row (#9829)

# Description
This command also flat-maps and doesn't create a table like `split
column`

We should probably reconsider the flatmap behavior like in #9739
but for the #9812 hotfix this is an unwelcome breaking change.

# User-Facing Changes
None

# Tests + Formatting
- Fix signature of `split row`
- Add test for output signature

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

@@ -45,5 +45,14 @@ fn to_row() {
         ));
 
         assert!(actual.out.contains('5'));
+
+        let actual = nu!(r#"
+                def foo [a: list<string>] {
+                    $a | describe
+                }
+                foo (["a b", "c d"] | split row " ")
+            "#);
+
+        assert!(actual.out.contains("list<string>"));
     })
 }