fix: also set ssrAppId for first Vue app when ssrAttribute exists (#563)

This reverts commit 3057828715.

examples/ssr/App.js

@@ -66,8 +66,7 @@ export default function createApp () {
     ]
   })
 
-  const app = new Vue({
-    router,
+  const App = ({
     metaInfo () {
       return {
         title: 'Boring Title',
@@ -157,6 +156,11 @@ export default function createApp () {
     </div>`
   })
 
+  const app = new Vue({
+    router,
+    render: h => h(App)
+  })
+
   const { set } = app.$meta().addApp('custom')
   set({
     bodyAttrs: { class: 'custom-app' },