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/src/strings/split/row.rs

@@ -18,7 +18,10 @@ impl Command for SubCommand {
         Signature::build("split row")
             .input_output_types(vec![
                 (Type::String, Type::List(Box::new(Type::String))),
-                (Type::List(Box::new(Type::String)), Type::Table(vec![])),
+                (
+                    Type::List(Box::new(Type::String)),
+                    (Type::List(Box::new(Type::String))),
+                ),
             ])
             .allow_variants_without_examples(true)
             .required(