define size = 0 when data is None, will prevent len be use update_data and keep the consistency of getData() and setData().

Other alternative should monkey patch update_data or avoid his usage removing "if/else". 



git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@29036 20353a03-c40f-0410-a6d1-a30d3c3de9de