检查ajax回调是否为空

        $http({
            url: "php/add.php",
            method: "POST",
            data: {
                'uId': this.uId,
            },
            headers: {
                'Content-Type': 'application/x-www-form-urlencoded'
            }
        }).success(function (data, status, headers, config) {

            if (data == "") {}else {
                alert('not empty');

            }

        });

I check my network tab in chrome's console, nothing has been echo, but the alert still pop up. Why?