Don't put invalid post categories into database

When creating a form there is now a post category question, although
it's only compulsory for visitor forms. However even if it is not filled
because the question has a default value representing "no data" we ended
up storing that value in the database, and the default value is an
invalid post category for our database schema. This didn't seem to do
any real harm (Hotwire simply displayed a blank) but was at best confusing when
examining the underlying column data. This change checks for the default
"no data" value and sets the database column null in that case instead.