tp3 判断数据库值为null

/**
 * 初始化车辆信息
 */
public function initCar() {
    $car = M('car');
    $edit_data = [
        'begin_date' => '2020-01-01 00:00:00',
    ];
    $flag = $car->where(['begin_date'=>['exp','is null']])->save($edit_data);
    $this->json->ok($flag);
}