Fixed #7582: Removed checks that prevented null=True ForeignKey fields from...

Fixed #7582: Removed checks that prevented null=True ForeignKey fields from being sortable in Admin.  Post queryset-refactor there seems no reason to disallow this.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@9080 bcc190cf-cafb-0310-a4f2-bffc1f526a37