[1.2.X] Fixed #13815 -- Ensure that reverse exclude lookups on nullable...

[1.2.X] Fixed #13815 -- Ensure that reverse exclude lookups on nullable foreign keys exclude null values. Thanks to bpeschier for the report and patch.

Backport of r15458 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15460 bcc190cf-cafb-0310-a4f2-bffc1f526a37