do no remove message when there is no messages

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@684 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/CMFActivity/Activity/SQLDict.py

@@ -64,6 +64,7 @@ class SQLDict(RAMDict):
     # Erase all messages in a single transaction
     uid_list = activity_tool.SQLDict_readUidList(path=m.object_path, method_id=m.method_id,processing_node=None)
     uid_list = map(lambda x:x.uid, uid_list)
+    if len(uid_list)>0:
       activity_tool.SQLDict_delMessage(uid = uid_list) 
     
   # Registration management