Forced blank=True for NullBooleanField to allow it to pass model validation with a value of None. Refs #13071.
Forced blank=True for NullBooleanField to allow it to pass model validation with a value of None. Refs #13071.

git-svn-id: 554f83ef17aa7291f84efa897c1acfc5d0035373@12744 bcc190cf-cafb-0310-a4f2-bffc1f526a37

svn2github/django,svn2github/django,svn2github/django