Fixed #4653 -- Improved the logic to decide when to include (and select as

initial value) the blank choice for a model field with choices. Thanks to
Ilya Semenov for persisting with this.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@6733 bcc190cf-cafb-0310-a4f2-bffc1f526a37