Request #7656 - Migration DocmanV1 fails when UGROUP_NONE or UGROUP_ANONYMOUS

Problem was coming from the fact that ugroup_db_get_dynamic_members can
in some cases (UGROUP_NONE and UGROUP_ANONYMOUS) return an empty SQL
query. It was not previously catched by the code leading to a fatal
error (particularly on import from DocmanV1). Empty sql query is now
catched and an empty data access result is returned in this case.

Change-Id: I60cb3b69bbdd842348ed46bff9df75a8dbe02c33
Merge commit 'refs/changes/20/3320/3' of ssh://gerrit.tuleap.net:29418/tuleap into stable

* ssh://gerrit.tuleap.net:29418/tuleap:
  Request #7656 - Migration DocmanV1 fails when UGROUP_NONE or UGROUP_ANONYMOUS
This is Tuleap 7.7.99.62

Change-Id: If40e63f96dfdc18bd2f25fde6a0bd263cf5b907f