Work around hhvm bug in redis job queue

The hhvm redis client returns false instead of null.  This caused
JobQueueRedis to get stuck in an infinite loop.  This works around the
difference by catching null as a signal for no more jobs.

It can be reverted when https://github.com/facebook/hhvm/pull/3127/ is
in all versions of hhvm we expect to run MediaWiki.

Bug: 67622
Change-Id: I9bbad42f36a80635097b8e0140b48b6492b2f0f5
Merge "Work around hhvm bug in redis job queue"