check that the modified name is not none. This improve a lot the perf. Previously this mistake make some pages to take 3 minutes to be totally loaded. Now it's about one second or less.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@23632 20353a03-c40f-0410-a6d1-a30d3c3de9de