Fix request #5559: Fatal error with elastic search in system events

Change-Id: If24ace86b71bd923c65556a69671d02b57daded9
Merge commit 'refs/changes/26/1426/1' of http://gerrit.tuleap.net/tuleap into tuleap-stable-master

* http://gerrit.tuleap.net/tuleap:
  Fix request #5559: Fatal error with elastic search in system events

Change-Id: I1913953d8a49caecc68c8e2214e4b85af35c3169
Tuleap 6.6.99.32

Change-Id: Iae66b75a7357a2d1af96f9b15bbb8defe5154ef6