there must not have a None tree id if no oject exists at root of HBTRree

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@25479 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/HBTreeFolder2/HBTreeFolder2.py

@@ -386,7 +386,7 @@ class HBTreeFolder2Base (Persistent):
         """
         if htree is None:
           htree = self._htree
-          btree_list = [None,]
+          btree_list = []
         else:
           btree_list = []
         for obj_id in htree.keys():