Fix #13864: Removed database error raised when force_update is requsted on...

Fix #13864: Removed database error raised when force_update is requsted on save of an inherited model with no fields of its own. Thanks fva, gregmuellegger, and markb1.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17088 bcc190cf-cafb-0310-a4f2-bffc1f526a37