Fix #3213 Rest arg is not optional (#3303)

crates/nu-engine/src/whole_stream_command.rs

@@ -137,6 +137,13 @@ impl WholeStreamCommand for Arc<Block> {
                     UntaggedValue::Table(elements).into_value(Span::new(start, end)),
                 );
             }
+        } else if block.params.rest_positional.is_some() {
+            //If there is a rest arg, but no args were provided,
+            //we have to set $rest to an empty table
+            ctx.scope.add_var(
+                "$rest",
+                UntaggedValue::Table(Vec::new()).into_value(Span::new(0, 0)),
+            );
         }
         if let Some(args) = evaluated.args.named {
             for named in &block.params.named {

tests/shell/pipeline/commands/internal.rs

@@ -278,6 +278,19 @@ fn run_custom_command_with_rest_and_flag() {
     assert_eq!(actual.out, r#"["world","hello","yay"]"#);
 }
 
+#[test]
+fn run_custom_command_with_empty_rest() {
+    let actual = nu!(
+        cwd: ".",
+        r#"
+            def rest-me-with-empty-rest [...rest: string] { echo $rest }; rest-me-with-empty-rest
+        "#
+    );
+
+    assert_eq!(actual.out, r#""#);
+    assert_eq!(actual.err, r#""#);
+}
+
 #[test]
 fn set_variable() {
     let actual = nu!(