I have a question on If statement while comparision with NULL value.

I have a trigger on a table which fires before insert for each row. In that trigger I am setting the default values for inserted date, inserted by etc. All that I am doing is to check if the passed value is null and if it is null then set it to a default value. However ELSE part of the statement is being executed. Can some body help me to identify what the issue is.

I am inserting a row thru PHPMyAdmin and below is the insert statement generated.

INSERT INTO `generic`.`sequence_generator` (
`unq_id` ,
`tbl_nm` ,
`max_value_plus_one` ,
`effctv_dttm` ,
`trmntd_dttm` ,
`insrtd_by` ,
`insrtd_dttm` ,
`last_updtd_by` ,
`last_updtd_dttm`
)
VALUES (
'', 'mmm', '1', '', NULL , '', '', '', ''
);

The part that is not working. This is extracted from the trigger.

if new.effctv_dttm is null
or new.effctv_dttm = ''
or length(new.effctv_dttm) = 0
OR length(new.effctv_dttm) = NULL
then
set new.effctv_dttm = current_timestamp;
else
set new.effctv_dttm = '2000-01-01 00:00:00';
end if;