fix no client QUIT on shutdown when it is not connected

[DoobleD]

Not telling redis client to quit on shutdown when it is not connected.

Fixes the following critical error when using graceful_shutdown=true in smtp.ini and sending SIGINT to Haraka:

[NOTICE] [-] [core] SIGINT received
[INFO] [-] [server] Shutting down listeners
[INFO] [-] [server] Workers closed. Shutting down master process subsystems
[INFO] [-] [outbound] Shutting down temp fail queue
[INFO] [-] [plugins] Shutting down
[CRIT] [-] [core] Error: The client is closed
[CRIT] [-] [core]     at RedisSocket.quit (/opt/Haraka/node_modules/@redis/client/dist/lib/client/socket.js:70:19)
[CRIT] [-] [core]     at Commander.QUIT (/opt/Haraka/node_modules/@redis/client/dist/lib/client/index.js:260:71)
[CRIT] [-] [core]     at exports.shutdown (/opt/Haraka/node_modules/haraka-plugin-redis/index.js:153:24)
[CRIT] [-] [core]     at exports.shutdown_plugins (/opt/Haraka/plugins.js:240:43)
[CRIT] [-] [core]     at process.<anonymous> (/opt/Haraka/plugins.js:248:17)
[CRIT] [-] [core]     at process.emit (node:events:530:35)
[CRIT] [-] [core]     at Server._graceful (/opt/Haraka/server.js:237:21)
[CRIT] [-] [core]     at process.processTicksAndRejections (node:internal/process/task_queues:95:5)

Checklist:

• docs updated
• tests updated
• Changes.md updated
• package.json.version bumped