Do not allow to export data when there a no data to export.
Related to sla #2759
Merge commit 'refs/changes/87/287/1' of http://gerrit.tuleap.net/tuleap into stable

* http://gerrit.tuleap.net/tuleap:
  Do not allow to export data when there a no data to export. Related to sla #2759
This is Tuleap 5.11.99.14