refactor(editor): refactor page note empty checker (#9570)

Close [BS-2320](https://linear.app/affine-design/issue/BS-2320/内容为空的状态判断)

blocksuite/affine/model/src/blocks/note/note-model.ts

@@ -111,6 +111,17 @@ export class NoteBlockModel
     if (!this._isSelectable()) return false;
     return super.intersectsBound(bound);
   }
+
+  override isEmpty(): boolean {
+    if (this.children.length === 0) return true;
+    if (this.children.length === 1) {
+      const firstChild = this.children[0];
+      if (firstChild.flavour === 'affine:paragraph') {
+        return firstChild.isEmpty();
+      }
+    }
+    return false;
+  }
 }
 
 declare global {