make sure also inventory is not None

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@11609 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5/Document/InventoryLine.py

@@ -113,7 +113,7 @@ class InventoryLine(DeliveryLine):
         # Make sure inventory is defined somewhere (here or parent)
         _marker = []
         inventory = getattr(aq_base(self), 'inventory', _marker)
-        if inventory is not _marker:
+        if inventory is not _marker and inventory is not None:
           return inventory
         return quantity
       else: