rest: shell/output does not fail if command was never run

skip including client in the listing if command never run or it did not finished
yet.

src/rest.c

@@ -781,7 +781,7 @@ static int og_cmd_run_get(json_t *element, struct og_msg_params *params,
 		struct og_client *cli;
 
 		cli = og_client_find(params->ips_array[i]);
-		if (!cli)
+		if (!cli || !cli->shell.tstamp)
 			continue;
 
 		object = json_object();