Corrected the acquisition value copying in accessor (2)

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@1312 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5Type/Base.py

@@ -229,7 +229,7 @@ class Base( CopyContainer, PortalContent, Base18, ActiveObject, ERP5PropertyMana
         value = method() # We should add depends here XXXXXX
                          # There is also a strong risk here of infinite loop
       if copy_value:
-        if not getattr(self, storage_id, None):
+        if getattr(self, storage_id, None) is None:
           # Copy the value if it does not already exist as an attribute of self
           # Like in the case of orders / invoices
           setattr(self, storage_id, value)