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/StudentNameManager.vue

@@ -43,7 +43,7 @@
         >
           稍后设置
         </v-btn>
-        <v-spacer/>
+        <v-spacer />
         <v-btn
           :disabled="!selectedName || saving"
           :loading="saving"