timestamp and null value

Hi,

I don't succeed in placing null values in a timestamp column.
This is an example :

create table entite (dt_creation timestamp null , dt_suppression timestamp);

insert into entite values ('', '1999-11-08');
ERROR: Bad timestamp external representation ''

Ok, that's my problem. Please note the fact that my "insert" command line
could not be modified, cause it's provided by a script, and I cannot modify
the script. I can't type for example :
insert into entite (dt_suppression) values ('1999-11-08');
wich would work.

I can't write it by hand, because I have more than 8,000,000 entries.

So if i can solve this problem just by modifying my "create table" line, it
would be great.

Thanks,
best regards,

Nicolas