Created by: gabegorelick
By returning null, a warning won't be generated about unhandled promise.
I'm not totally sure how to reproduce this, but I think it happens when using a version of ioredis that returns Bluebird promises. I do not have a minimal test case, but I can reproduce it reliably. This change fixes it 100% of the time.
Alternatively, https://github.com/OptimalBits/bull/blob/c0ee6be03c6d47d60465d2bae0d2566d5bf0d661/lib/queue.js#L839 should potentially return the promise. But I'm not sure if not returning it is intentional.