Allow empty list inputs in group-by and return empty record (#10730)

# Description

Changed `group-by` behavior to accept empty list as input and return an
empty record instead of throwing an error. I also replaced
`errors_if_input_empty()` test to reflect the new expected behavior.

See #10713 

# User-Facing Changes
`[] | group-by` or `[] | group-by a` now returns empty record


# Tests + Formatting
1 test for emptied table i.e. list

---------

Signed-off-by: Oscar <71343264+0scvr@users.noreply.github.com>

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

@@ -97,9 +97,10 @@ fn errors_if_column_not_found() {
 }
 
 #[test]
-fn errors_if_input_empty() {
-    let actual = nu!("group-by date");
-    assert!(actual.err.contains("expected table from pipeline"));
+fn group_by_on_empty_list_returns_empty_record() {
+    let actual = nu!("[[a b]; [1 2]] | where false | group-by a");
+    assert!(actual.err.is_empty());
+    assert!(actual.out.contains("empty record"));
 }
 
 #[test]