TypeError: Cannot read property 'zcard' of null

```
TypeError: Cannot read property 'zcard' of null
at Queue.card (/Users/andrew/Code/send/node_modules/kue/lib/kue.js:513:14)
at Queue.inactiveCount (/Users/andrew/Code/send/node_modules/kue/lib/kue.js:616:17)
at _ (/Users/andrew/Code/send/node_modules/kue/lib/http/routes/json.js:318:19)
at exports.stats (/Users/andrew/Code/send/node_modules/kue/lib/http/routes/json.js:41:3)
at Layer.handle [as handle_request] (/Users/andrew/Code/send/node_modules/kue/node_modules/express/lib/router/layer.js:95:5)
at next (/Users/andrew/Code/send/node_modules/kue/node_modules/express/lib/router/route.js:131:13)
at /Users/andrew/Code/send/node_modules/kue/lib/http/middleware/provides.js:11:36
at Layer.handle [as handle_request] (/Users/andrew/Code/send/node_modules/kue/node_modules/express/lib/router/layer.js:95:5)
at next (/Users/andrew/Code/send/node_modules/kue/node_modules/express/lib/router/route.js:131:13)
at Route.dispatch (/Users/andrew/Code/send/node_modules/kue/node_modules/express/lib/router/route.js:112:3)
at Layer.handle [as handle_request] (/Users/andrew/Code/send/node_modules/kue/node_modules/express/lib/router/layer.js:95:5)
at /Users/andrew/Code/send/node_modules/kue/node_modules/express/lib/router/index.js:277:22
at Function.process_params (/Users/andrew/Code/send/node_modules/kue/node_modules/express/lib/router/index.js:330:12)
at next (/Users/andrew/Code/send/node_modules/kue/node_modules/express/lib/router/index.js:271:10)
at SendStream.error (/Users/andrew/Code/send/node_modules/kue/node_modules/express/node_modules/serve-static/index.js:120:7)
at emitOne (events.js:77:13)
at SendStream.emit (events.js:169:7)
at SendStream.error (/Users/andrew/Code/send/node_modules/kue/node_modules/express/node_modules/send/index.js:245:17)
at SendStream.onStatError (/Users/andrew/Code/send/node_modules/kue/node_modules/express/node_modules/send/index.js:356:12)
at next (/Users/andrew/Code/send/node_modules/kue/node_modules/express/node_modules/send/index.js:630:16)
at onstat (/Users/andrew/Code/send/node_modules/kue/node_modules/express/node_modules/send/index.js:619:14)
at FSReqWrap.oncomplete (fs.js:82:15)
```

I get this error pretty consistently under the following conditions:
1. Have kui ui open in a browser window
2. perform graceful shutdown

I'm not certain if it is expected for Queue.client to be null. If it is, then `Queue.prototype.card` should first check if `this.client` exists before calling `this.client.card`. Same with `Queue.prototype.cardByType`

Start in lib/kue.js at Queue.prototype.card and Queue.prototype.cardByType, then trace the stats path in lib/http/routes/json.js around lines 318 and 41. Reproduce with the Kue UI open during graceful shutdown and verify that the stats request no longer raises the reported TypeError; add or update a regression test if the repository has coverage for this lifecycle.

Written by the indexing model from the issue text.