[1.3.X] Fixed #15776 - delete regression in Django 1.3 involving nullable foreign keys

Many thanks to aaron.l.madison for the detailed report and to emulbreh for
the fix.

Backport of [16295] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.3.X@16296 bcc190cf-cafb-0310-a4f2-bffc1f526a37