Inserting NULL into Integer column

Hi,

in mysql I was able to make an insert such as:

INSERT INTO TABLE (integervariable) VALUES ('')

and have it either insert that variable, or insert the default if it had
been assigned.  In postgresql it gives and error every time that this is
attempted.  Since I have so many queries that do this on my site already, is
there any way to set up a table so that it just accepts this sort of query?

Thanks,
Jeremy