Fixed #12819. Fixed a bug with caching values of nullable 1to1 fields. Thanks,...

Fixed #12819. Fixed a bug with caching values of nullable 1to1 fields. Thanks, s.angel@twidi.com for the initial patch, and Alex Gaynor for the tests.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12543 bcc190cf-cafb-0310-a4f2-bffc1f526a37