Fix catalog::columns::is_nullable to handle valid NULL.
Fix catalog::columns::is_nullable to handle valid NULL.

SQLColumns is allowed to report IS_NULLABLE as NULL.

nanodbc/nanodbc,nanodbc/nanodbc,mcg1969/nanodbc,ChrisBFX/nanodbc,norsync/nanodbc,ChrisBFX/nanodbc,norsync/nanodbc,mcg1969/nanodbc