Fix bug in Job state method which would fail if no callback was provided

Specifically this resulted in a lot of errors in our logs that looked like this:

    TypeError: undefined is not a function
        at Job.<anonymous> (/omitted.../node_modules/kue/lib/queue/job.js:494:87)
        at try_callback (/omitted.../node_modules/redis/index.js:581:9)
        at RedisClient.return_reply (/omitted.../node_modules/redis/index.js:671:13)
        at ReplyParser.<anonymous> (/omitted.../node_modules/redis/index.js:313:14)
        at ReplyParser.EventEmitter.emit (events.js:95:17)
        at ReplyParser.send_reply (/omitted.../node_modules/redis/lib/parser/javascript.js:300:10)
        at ReplyParser.execute (/omitted.../node_modules/redis/lib/parser/javascript.js:189:22)
        at RedisClient.on_data (/omitted.../node_modules/redis/index.js:536:27)
        at Socket.<anonymous> (/omitted.../node_modules/redis/index.js:91:14)
        at Socket.EventEmitter.emit (events.js:95:17)

(omitted some particulars of my setup)

This commit and pull request fixes that issue.