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/auth/AlternativeCodeDialog.vue

@@ -20,7 +20,7 @@
       </v-alert>
     </v-card-text>
     <v-card-actions>
-      <v-spacer/>
+      <v-spacer />
       <v-btn
         v-if="showCancel"
         variant="text"