replace return None by ""

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@14735 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5SyncML/XMLSyncUtils.py

@@ -296,7 +296,7 @@ class XMLSyncUtilsMixin(SyncCode):
     This just allow to get the xml of the object
     """
     if xml_mapping in ['None', None]:
-      return
+      return ''
     xml_method = None
     xml = ""
     if xml_mapping is not None: