[1.2.X] Changed unique validation in model formsets to ignore None values, not...

[1.2.X] Changed unique validation in model formsets to ignore None values, not just omit them. Thanks claudep!

Backport of r14193 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14194 bcc190cf-cafb-0310-a4f2-bffc1f526a37