dispatch: change log level to debug for rpc list empty list

when dynamically loading/querying the dispatcher list at runtime, empty dslist logs as error which misleads log monitoring tools looking for real errors

(cherry picked from commit ca230986b7f4ea84746c47df177ac21b19fd70a3)

src/modules/dispatcher/dispatcher.c

@@ -1291,7 +1291,7 @@ static void dispatcher_rpc_list(rpc_t *rpc, void *ctx)
 	int ds_list_nr = ds_get_list_nr();
 
 	if(ds_list == NULL || ds_list_nr <= 0) {
-		LM_ERR("no destination sets\n");
+		LM_DBG("no destination sets\n");
 		rpc->fault(ctx, 500, "No Destination Sets");
 		return;
 	}