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

PHPBB3-12448
[ticket/12448] Fix null columns for postgres

PHPBB3-12448
[ticket/12448] Fix null columns for MS SQL

PHPBB3-12448
Merge pull request #2651 from nickvergessen/ticket/12448

[ticket/12448] Allow null as default value for columns
Merge branch 'develop-ascraeus' into develop

# By Joas Schilling
# Via Dhruv Goel (1) and Joas Schilling (1)
* develop-ascraeus:
  [ticket/12448] Fix null columns for MS SQL
  [ticket/12448] Fix null columns for postgres
  [ticket/12448] Allow null as default value for columns