Compare a value with null values rather than None. This does not fix...

Compare a value with null values rather than None. This does not fix testConstraint, but the test is wrong in my opinion.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@17302 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5Type/Accessor/Base.py

@@ -188,4 +188,4 @@ class Tester(Method):
       self._storage_id = storage_id
 
     def __call__(self, instance, *args, **kw):
-      return getattr(aq_base(instance), self._storage_id, None) is not None
+      return getattr(aq_base(instance), self._storage_id, None) not in self._null