[1.1.X] Fixed #12240 - select_related doesn't work correctly when mixing...

[1.1.X] Fixed #12240 - select_related doesn't work correctly when mixing nullable and non-nullable keys
  
Thanks to embe for report and Alex for fix.

Backport of 12719 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12720 bcc190cf-cafb-0310-a4f2-bffc1f526a37