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/root/root-block-model.ts

@@ -22,6 +22,22 @@ export class RootBlockModel extends BlockModel<RootBlockProps> {
       });
     });
   }
+
+  /**
+   * A page is empty if it only contains one empty note and the canvas is empty
+   */
+  override isEmpty() {
+    let numNotes = 0;
+    let empty = true;
+    for (const child of this.children) {
+      empty = empty && child.isEmpty();
+
+      if (child.flavour === 'affine:note') numNotes++;
+      if (numNotes > 1) return false;
+    }
+
+    return empty;
+  }
 }
 
 export const RootBlockSchema = defineBlockSchema({