fix：加个平均时长判断；

Reviewed-on: #420

src/renderer/src/views/classTask/container/classOverview/timeAnalyse.vue

@@ -152,6 +152,7 @@ const getXValue = () => {
 
 watch(() => useOverview.tableList, () => {
   const time = useOverview.tableList.map(item => Number(item.timelength))
+  if(time.length === 0) return;
   const avatarTime = time.reduce((acc, cur) => {
     return acc + cur
   },0) / time.length