Fixed JobQueueAggregatorRedis to handle empty collections

* Previously it would keep polling if the hash was empty since
  empty and non-existing collections are the same in Redis.
* Also turned off the pointless PHP serialization.
* Bumped the cache version key; this change should be deployed
  in all wikis at once, along with updating the runners.

Change-Id: I33dc55bc579bf678f14a5383e9e2c3c35231d599