Sometimes cell.getContributionShare can return None. This script fails,
as it is not possible to add None with script. Thanks to this change
script will not fail in (possibly) wrongly configured environment.


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@31928 20353a03-c40f-0410-a6d1-a30d3c3de9de