Avoid making a derivative request to Special:RunJobs when the job queue is empty

Checking that the job queue is nonempty is cheap, and when the result is
negative (that is, when the queue is actually empty), it spares from having to
launch a derivative request to Special:RunJobs for job processing. The
derivative request is not cheap, because processing it requires having an
additional full instance of the application. This patch adds the check.

Bug: 60210
Change-Id: Icb95c35f4a8a3c9d4b5aff97fdfabf596d05940a