Fix "Index out of bounds" when input to the group-by filter is empty. #4369 (#4382)

* Fix "index out of bounds" when input to group-by is empty #4369

* Fix formatting #4369

* Adds test for empty input

Co-authored-by: Ray Henry <ray.henry@thermofisher.com>

crates/nu-command/src/filters/group_by.rs

@@ -95,8 +95,6 @@ pub fn group_by(
     let mut keys: Vec<Result<String, ShellError>> = vec![];
     let mut group_strategy = Grouper::ByColumn(None);
 
-    let first = values[0].clone();
-
     if values.is_empty() {
         return Err(ShellError::SpannedLabeledError(
             "expected table from pipeline".into(),
@@ -105,6 +103,8 @@ pub fn group_by(
         ));
     }
 
+    let first = values[0].clone();
+
     let value_list = Value::List {
         vals: values.clone(),
         span: name,

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

@@ -97,3 +97,17 @@ fn errors_if_block_given_evaluates_more_than_one_row() {
         assert!(actual.err.contains("Unknown column"));
     })
 }
+
+#[test]
+fn errors_if_input_empty() {
+    Playground::setup("group_by_empty_test", |dirs, _sandbox| {
+        let actual = nu!(
+            cwd: dirs.test(), pipeline(
+            r#"
+            group-by date
+        "#
+        ));
+
+        assert!(actual.err.contains("expected table from pipeline"));
+    });
+}