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/pages/debug-init.vue

@@ -26,7 +26,7 @@
                 label="server.authDomain"
               />
             </v-form>
-            <v-divider class="my-4"/>
+            <v-divider class="my-4" />
 
             <v-btn
               class="me-2"