test: add exit without args test

src/builtins/exit.rs

@@ -17,4 +17,9 @@ mod tests {
     fn test_exit() {
         assert_eq!(Exit.execute(vec!["arg".to_string()]), Err(ShellError::Exit))
     }
+
+    #[test]
+    fn test_exit_without_args() {
+        assert_eq!(Exit.execute(vec![]), Err(ShellError::Exit))
+    }
 }