Fix gnt-job list with empty timestamps

Fix gnt-job list with empty timestamps

In case the job object doesn't have a timestamp (which is a separate
issue), the listing should not break. We fix this by changing the
FormatTimstamp function itself to return '?' in case the timestamp
doesn't look good (note that it still can break if non-integers are
returned, but this is unlikely).

Reviewed-by: imsnah