[ticket/12448] Allow null as default value for columns

PHPBB3-12448

Tested on:
- [x] MS SQL
- [x] MS SQL native: https://bamboo.phpbb.com/browse/PHPBB3-MSSQLNATIVE-24
- [x] MySQL (Travis)
- [x] MySQLi (Travis)
- [x] Oracle
- [x] PostgreSQL (Travis)
- [x] SQLite2: https://bamboo.phpbb.com/browse/PHPBB3-PR-2930
- [x] SQLite3 (Travis)