为什么存储过程执行'null'而不是null？

Question

下面的参数是需要传递的参数

    IN CUSTOMERID_ INT(11),

    IN STAFFID_ INT(11),
    IN ACTIVITYDETAILS_ TEXT,

    IN RELATIONSHIPID1_ INT(11),
    IN FULLNAME1_ VARCHAR(100), 
    IN PHONENUM1_ VARCHAR(20), 
    IN FIELD11_ VARCHAR(100),
    IN FIELD21_ VARCHAR(100),
    IN POSTCODE1_ INT(5),
    IN CITY1_ VARCHAR(45),
    IN STATEID1_ TINYINT(1),
    IN COUNTRYID1_ INT(11),
    IN RELATIONSHIPYEAR1_ INT(3),

    IN RELATIONSHIPID2_ INT(11),
    IN FULLNAME2_ VARCHAR(100), 
    IN PHONENUM2_ VARCHAR(20), 
    IN FIELD12_ VARCHAR(100),
    IN FIELD22_ VARCHAR(100),
    IN POSTCODE2_ INT(5),
    IN CITY2_ VARCHAR(45),
    IN STATEID2_ TINYINT(1),
    IN COUNTRYID2_ INT(11),
    IN RELATIONSHIPYEAR2_ INT(3)

当我调用该过程时，其执行过程如下，

call lms.create_emergency(50, 7, 'testtt', 1, 'ali', '0145234568', 'lot786', 'street', 40400, 'shah alam', 1, 1, null, null, 'null', 'null', 'null', 'null', null, 'null', null, null, null);

为什么存储过程对varchar数据类型执行'null'而不是null？

如何避免这种情况？