[1.0.X] Fixed #9640, #10549: BooleanFields with choices, a default, and...

[1.0.X] Fixed #9640, #10549: BooleanFields with choices, a default, and null=False now correctly doesn't generate a blank option. Backport of r10500 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@10502 bcc190cf-cafb-0310-a4f2-bffc1f526a37