Fixed #8528 -- Ensure that null values are displayed as a filtering option in the admin if a field allows nulls. Thanks to StevenPotter for the report, and oyvind, marcob, Simon Meers and Julien Phalip for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15649 bcc190cf-cafb-0310-a4f2-bffc1f526a37