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/framework/store/src/model/block/block-model.ts

@@ -70,9 +70,9 @@ export class BlockModel<
 
   id!: string;
 
-  isEmpty = computed(() => {
-    return this._children.value.length === 0;
-  });
+  isEmpty() {
+    return this.children.length === 0;
+  }
 
   keys!: string[];
 

blocksuite/framework/store/src/model/blocks/blocks.ts

@@ -192,7 +192,7 @@ export class Blocks {
   }
 
   get isEmpty() {
-    return Object.values(this._blocks.peek()).length === 0;
+    return this.root?.isEmpty() ?? true;
   }
 
   get loaded() {