fix null 显示成 invalid date

src/features/OnlinePractice/OnlinePracticeRecordList.vue

@@ -86,7 +86,7 @@ export default {
       );
     },
     formatTime(ms) {
-      return moment.utc(ms).format("HH:mm:ss") || "";
+      return (ms && moment.utc(ms).format("HH:mm:ss")) || "";
     }
   },
   computed: {