Fix #17879: Corrected regression in python (inherited by yaml and json)...

Fix #17879: Corrected regression in python (inherited by yaml and json) serializer that prevented serializing model instances with null FK ref to a model when serializing with natural keys. Thanks danfairs and tmitchell.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@17685 bcc190cf-cafb-0310-a4f2-bffc1f526a37