php empty()奇怪现象

代码：

      $userid =  $this->auth->id;
      var_dump(empty($userid));
      var_dump(empty($this->auth->id));

        //用户登录
        if(empty($this->auth->id)){
            return sys_return(
                \app\api\model\Code::FAIL,
                '未检测到登录用户'
            );
        }

返回：

bool(false)
bool(true)
{"code":1001,"msg":"未检测到登录用户","data":[],"time":1621822971}