MT#9709 get_all_cluster_set*: fix 2437330d4e39

This was producing empty values at kamailio.cfg template on CE
with a cluster_set of central type

Thanks to Anibal Cañada for the bugreport

Change-Id: Ib11295611859ef2828dc1d2e6900bf201c20ad7a
(cherry picked from commit 4d297ff3fc434ee822d467d41aa4b2ed8e0d3d36)