Fix request #6804: Notice prevents REST /projects:id from returning 404 when in debug mode.

Now returning null if group_id does not exits to keep current behaviour

Change-Id: I5ea84fe314b5bf3ce30efe7ed683880b241470fd
Merge commit 'refs/changes/76/2376/1' of ssh://gerrit.tuleap.net:29418/tuleap into tuleap-stable-master

* ssh://gerrit.tuleap.net:29418/tuleap:
  Fix request #6804: Notice prevents REST /projects:id from returning 404 when in debug mode.

Change-Id: I97c956dfa0b0668af034c20722acbb8bd64a8a1a
Tuleap 6.12.99.107

Change-Id: I1acc9401e4fddd192eb50f930766f0790a9c5b94