Remove support for assigning None to a SplitField

This behavior wasn't documented and didn't fully work: it breaks
as soon as you try to save a model with a `None` value.

model_utils/fields.py

@@ -208,9 +208,6 @@ def __init__(self, field):
     def __get__(self, instance, owner):
         if instance is None:
             raise AttributeError('Can only be accessed via an instance.')
-        content = instance.__dict__[self.field.name]
-        if content is None:
-            return None
         return SplitText(instance, self.field.name, self.excerpt_field_name)
 
     def __set__(self, obj, value):

tests/test_fields/test_split_field.py

@@ -53,10 +53,6 @@ def test_access_via_class(self):
         with self.assertRaises(AttributeError):
             Article.body
 
-    def test_none(self):
-        a = Article(title='Some Title', body=None)
-        self.assertEqual(a.body, None)
-
     def test_assign_splittext(self):
         a = Article(title='Some Title')
         a.body = self.post.body