主控台-未选择相机的时候，增加请选择相机的提示

src/views/dashboard/home/components/CameraInfo.vue

@@ -13,7 +13,8 @@
       />
     </div>
     <div class="video-block">
-      <LiveVideo :url="streamIp" />
+      <LiveVideo :url="streamIp" v-if="selectedCamera" />
+      <div v-else style="margin-top: 50px; text-align: center">请选择相机</div>
     </div>
     <div class="flex" style="width: 100%">
       <span class="algo-text">相关算法：</span>