Fixed #14119 -- fields_for_model no longer returns all fields when fields...

Fixed #14119 -- fields_for_model no longer returns all fields when fields parameter is the empty tuple. Thanks alexdutton!

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14199 bcc190cf-cafb-0310-a4f2-bffc1f526a37