request #7693: fatal error on PATCH route when milestone does not have artifact link field

Change-Id: I0480dd0d26f88eb4c17b4d1ffab1b62c7db22e0d
Merge commit 'refs/changes/64/3364/1' of ssh://gerrit.tuleap.net:29418/tuleap into tuleap-stable-master

* ssh://gerrit.tuleap.net:29418/tuleap:
  request #7693: fatal error on PATCH route when milestone does not have artifact link field

Change-Id: I336ed5271c193299ad14092ceedab5e119b55777
This is Tuleap 7.8.99.7

Change-Id: I5287c513f270f7c172c2f8d2889f10459cc2df86