corrected if best practice

functions/command_install.sh

@@ -19,7 +19,7 @@ elif [ "${gamename}" == "Unreal Tournament 99" ]; then
 	install_server_files.sh
 elif [ "${gamename}" == "Teamspeak 3" ]; then
 	install_ts3.sh
-elif [ ! -z "${appid}" ]; then
+elif [ -n "${appid}" ]; then
 	install_steamcmd.sh
 	install_server_files.sh
 fi