Fix bug #2950: getTrackerFields returning registered users in values field

fixing issue when there is no ugroups

Change-Id: Iff2ea61e42996fdd97c5e6d1770bbfb3840e9d78
Merge branch 'master' of tuleap.net:tuleap/tuleap/stable into stable

* 'master' of tuleap.net:tuleap/tuleap/stable:
  Do not include mediawiki stuff in PHP51
Merge commit 'refs/changes/48/348/3' of http://gerrit.tuleap.net/tuleap into stable

* http://gerrit.tuleap.net/tuleap:
  Fix bug #2950: getTrackerFields returning registered users in values field
This is Tuleap 5.12.99.13