rc: proper isset() usage in openvpn code

src/etc/rc.openvpn

@@ -72,7 +72,7 @@ if (file_exists('/var/run/booting')) {
 /* Input argument is a comma-separated list of gateway names, blank or "all". */
 $argument = trim($argv[1], " \n");
 
-if(is_array($config['openvpn']['openvpn-server']) || is_array($config['openvpn']['openvpn-client'])) {
+if (isset($config['openvpn']['openvpn-server']) || isset($config['openvpn']['openvpn-client'])) {
 	if (empty($argument) || $argument == "all") {
 		$argument = "all";
 		$log_text = "all";