test: add format time default test

src/builtins/time.rs

@@ -51,4 +51,10 @@ mod tests {
         let time = chrono::Utc.with_ymd_and_hms(6969, 4, 20, 4, 20, 9).unwrap();
         assert_eq!(format_time(time, "RFC3339"), "6969-04-20T04:20:09+00:00")
     }
+
+    #[test]
+    fn test_format_time_default() {
+        let time = chrono::Utc.with_ymd_and_hms(6969, 4, 20, 4, 20, 9).unwrap();
+        assert_eq!(format_time(time, ""), "04:20:09")
+    }
 }