Continue loop in worker when no job is returned

After a timeout, a queue can return nothing if no job is present. At this moment, the [worker quits](https://github.com/juriansluiman/SlmQueue/blob/master/src/SlmQueue/Worker/AbstractWorker.php#L71) which should not be the case. It should just do a `continue;` to get again in a new iteration of the loop.