If there is no attribute on a node, the an empty list will be adedd. So it's
required to check if the list corresponding to the current node is not empty
before applying attributes update.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@31526 20353a03-c40f-0410-a6d1-a30d3c3de9de