Do not crash when artifact has no last changeset

Change-Id: Iec12dc20d3ae451d2217ce5149fa81c237733e72
Merge commit 'refs/changes/65/465/3' of http://gerrit.tuleap.net/tuleap into stable

* http://gerrit.tuleap.net/tuleap:
  Do not crash when artifact has no last changeset
This is Tuleap 6.1.99.12