add isEmpty into pageContainer state

packages/app/src/client/services/PageContainer.js

@@ -52,6 +52,7 @@ export default class PageContainer extends Container {
       revisionId,
       revisionCreatedAt: +mainContent.getAttribute('data-page-revision-created'),
       path,
+      isEmpty: mainContent.getAttribute('data-page-is-empty'),
 
       createdAt: mainContent.getAttribute('data-page-created-at'),
       // please use useCurrentUpdatedAt instead