Fix notification deletion bug: save {} instead of [] when list is empty

When all notifications are deleted, the persistentNotifications array
becomes empty ([]). The backend doesn't accept an empty array as a
valid value, requiring an empty object ({}) instead. This fix modifies
both index.vue and UrgentTestDialog.vue to save {} when the
notification list is empty.

Co-authored-by: Sunwuyuan <88357633+Sunwuyuan@users.noreply.github.com>

src/components/attendance/AttendanceSidebar.vue

@@ -27,9 +27,9 @@
       <span style="white-space: nowrap">
         {{
           studentList.length -
-          attendance.absent.length -
-          attendance.late.length -
-          attendance.exclude.length
+            attendance.absent.length -
+            attendance.late.length -
+            attendance.exclude.length
         }}人
       </span>
     </h2>
@@ -45,8 +45,7 @@
       :key="'absent-' + index"
       class="gray-text"
     >
-      <span v-if="display.lgAndUp.value">{{ `${index + 1}. ` }}</span
-      ><span style="white-space: nowrap">{{ name }}</span>
+      <span v-if="display.lgAndUp.value">{{ `${index + 1}. ` }}</span><span style="white-space: nowrap">{{ name }}</span>
     </h3>
     <h2>
       <span style="white-space: nowrap">迟到</span>
@@ -60,8 +59,7 @@
       :key="'late-' + index"
       class="gray-text"
     >
-      <span v-if="display.lgAndUp.value">{{ `${index + 1}. ` }}</span
-      ><span style="white-space: nowrap">{{ name }}</span>
+      <span v-if="display.lgAndUp.value">{{ `${index + 1}. ` }}</span><span style="white-space: nowrap">{{ name }}</span>
     </h3>
     <h2>
       <span style="white-space: nowrap">不参与</span>
@@ -75,8 +73,7 @@
       :key="'exclude-' + index"
       class="gray-text"
     >
-      <span v-if="display.lgAndUp.value">{{ `${index + 1}. ` }}</span
-      ><span style="white-space: nowrap">{{ name }}</span>
+      <span v-if="display.lgAndUp.value">{{ `${index + 1}. ` }}</span><span style="white-space: nowrap">{{ name }}</span>
     </h3>
   </v-col>
 </template>