xinetd: fix syntax violation

see https://github.com/openwrt/packages/pull/16318#discussion_r687481110

Signed-off-by: Fritz D. Ansel <fdansel@yandex.ru>

net/xinetd/files/xinetd.init

@@ -71,7 +71,7 @@ config_cb() {
 			local value="$2"
 
 			# write out last list option if new list starts
-			if [ "$ListName" != "" -a "$ListName" != "$name" ]; then
+			if [ -n "$ListName" ] && [ "$ListName" != "$name" ]; then
 
 				echo -e "\t$ListName = $ListVals" >> $GENERATED_CONF_FILE