Disallow empty comments and comments with only whitespaces (#7156)

* fix: disallow empty comments

* fix: avoid full traversal to validate comment

* fix: text

* fix:review

* fix: review

shared/editor/nodes/Image.tsx

@@ -166,6 +166,8 @@ export default class Image extends SimpleImage {
           ["p", { class: "caption" }, 0],
         ];
       },
+      toPlainText: (node) =>
+        node.attrs.alt ? `(image: ${node.attrs.alt})` : "(image)",
     };
   }