[1.1.X] Fixed #12105: Corrected handling of isnull=False lookups in admin....

[1.1.X] Fixed #12105: Corrected handling of isnull=False lookups in admin. Thanks marcob, Travis Cline, gabrielhurley. 

r12795 backport from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12796 bcc190cf-cafb-0310-a4f2-bffc1f526a37