Fixed #29227 -- Allowed BooleanField to be null=True.

Thanks Lynn Cyrin for contributing to the patch, and Nick Pope for review.

tests/queries/models.py

@@ -676,7 +676,7 @@ class Student(models.Model):
 
 class Classroom(models.Model):
     name = models.CharField(max_length=20)
-    has_blackboard = models.NullBooleanField()
+    has_blackboard = models.BooleanField(null=True)
     school = models.ForeignKey(School, models.CASCADE)
     students = models.ManyToManyField(Student, related_name='classroom')