Adjust ModelField.null mappings now that Django-25320 is resolved

rest_framework/utils/field_mapping.py

@@ -233,14 +233,9 @@ def get_relation_kwargs(field_name, relation_info):
             # No further keyword arguments are valid.
             return kwargs
 
-        # Currently required for 1.9 master behavior,
-        # may be able to remove this with 1.9 alpha.
-        # See https://code.djangoproject.com/ticket/25320
-        null = model_field.null and not to_many
-
-        if model_field.has_default() or model_field.blank or null:
+        if model_field.has_default() or model_field.blank or model_field.null:
             kwargs['required'] = False
-        if null:
+        if model_field.null:
             kwargs['allow_null'] = True
         if model_field.validators:
             kwargs['validators'] = model_field.validators