antd a-select组件placeholder不显示解决办法和原因

       <a-select
          ref="select"
          v-model:value="formState.parentId"
          style="width: 100%"
          placeholder="请选择"
        >
        </a-select>
   setup(props, { emit }) {
    const formState = reactive<Form>({
      parentId: undefined,//不能为''或者null
    })