Fixed #10792 -- Ensured that ModelChoiceFields don't provide an empty option when the underlying field has blank=False and there is a default value available. Thanks to carljm for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10729 bcc190cf-cafb-0310-a4f2-bffc1f526a37