Avoid overloading IdGenerator if it is not provided

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@30526 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5Type/Core/Folder.py

+# -*- coding: utf-8 -*-
 ##############################################################################
 #
 # Copyright (c) 2002 Nexedi SARL and Contributors. All Rights Reserved.
@@ -490,6 +491,7 @@ class Folder(CopyContainer, CMFBTreeFolder, CMFHBTreeFolder, Base, FolderMixIn,
     # we may want to change all objects ids before migrating to new folder type
     # set new id generator here so that object created while migration
     # got a right id
+    if new_generate_id_method is not None:
       self.setIdGenerator(new_generate_id_method)
     if migration_generate_id_method not in (None, ''):
       tag = "%s/%s/migrate" %(self.getId(),migration_generate_id_method)