[1.0.X] Fixed #9862 -- For better SQL portability, don't specify "NULL" on...

[1.0.X] Fixed #9862 -- For better SQL portability, don't specify "NULL" on nullable columns when creating tables. Patch from Ian Kelly.

Columns are NULL by default, so we only need to use "NOT NULL" when we want
non-default behaviour.

Backport of r9703 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@9704 bcc190cf-cafb-0310-a4f2-bffc1f526a37