As far as I can see, this query:
qa_db_query_sub('UPDATE `qa_users` SET '.$field.' = $ WHERE userid = # ',
insert a string for $field when the passed $value is NULL.
How can I force the NULLing of the field?
Seems that I had a trim($value) somewhere that made the NULL an empty string.
So NULL is set in the DB using the query above!