Don't consider an empty queue as an error

When running the worker, for any empty queue, we will get `redigo: nil returned`.

This is because `BRPOPLPUSH` returns `null` when the queue is empty.
[There is an issue about that](https://code.google.com/p/redis/issues/detail?id=650).

This patch ignores this error, so we don't get the false impression to have an error in the worker when that is not the case.

**Note:** I wanted to write a test for this, but it would seem gospec doesn't support mocking, and didn't want to end up adding a new dependency for such a small fix.