[1.1.X] Restored pre-r10062 behavior allowing None from formfield_callback to...

[1.1.X] Restored pre-r10062 behavior allowing None from formfield_callback to exclude itself from the form

Backported from r12891 from trunk

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12892 bcc190cf-cafb-0310-a4f2-bffc1f526a37