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/DeviceAuthDialog.vue

@@ -41,9 +41,7 @@
           label="命名空间"
           prepend-inner-icon="mdi-identifier"
           variant="outlined"
-        >
-
-        </v-text-field>
+        />
 
         <v-text-field
           v-model="form.password"
@@ -51,9 +49,7 @@
           prepend-inner-icon="mdi-lock-outline"
           type="text"
           variant="outlined"
-        >
-
-        </v-text-field>
+        />
 
         <v-alert
           v-if="error"
@@ -77,7 +73,7 @@
       >
         取消
       </v-btn>
-      <v-spacer/>
+      <v-spacer />
       <v-btn
         :disabled="!form.namespace || authenticating"
         :loading="authenticating"