[1.0.X] Fixed #9649 -- Better error handling in model creation.

Previously, you could explicitly assign None to a non-null ForeignKey
(or other) field when creating the model (Child(parent=None), etc). We
now throw an exception when you do that, which matches the behaviour
when you assign None to the attribute after creation.

Thanks to ales.zoulek@gmail.com and ondrej.kohout@gmail.com for some
analysis of this problem.

Backport of r9983 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@9984 bcc190cf-cafb-0310-a4f2-bffc1f526a37