In the time it takes to process incoming log messages, the process we
are logging details for may exit. This means the cgroup data is no
longer available from '/proc'. Unfortunately, the way the code was
structured before, we never log _SYSTEMD_UNIT if we don't have this
cgroup information.
Add an else if case that allows the passed in unit_id to be logged even
if we couldn't capture cgroup information. This ensures a command like
`journalctl -u run-XXX` will return all log messages from a oneshot
process.