scheduler/job.c: Fix extensive logging in scheduler

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