jqueue: Don't return negative number for unchecked jobs when archiving

When the queue was empty, the calculation for unchecked jobs while
archiving would return -1. ``last_touched`` is set to 0, the job ID list
(``all_job_ids``) is empty. Calculating ``len(all_job_ids) -
last_touched - 1`` resulted in -1.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>