scheduler/job.c: Fix extensive logging in scheduler

Based on currently unknown trigger scheduler sometimes sets JobHistoryUpdate into past, which causes select() to timeout after one second.

It happens when job->file_time of a job without files to remove gets assigned to JobHistoryUpdate. If we check for job->num_files and assign the job->file_time only if there are any, we will fix extensive logging (and unneeded cupsd execution) in various places, e.g. cleaning jobs, expiring subscriptions, deleting temporary queues...

Fixes #604