dissect: if dissecting without udev, don't look for usec timestamp on db record

There will likely be none, hence don't bother.

This fixes an issue in systemd-gpt-auto-generator where we'll try to
wait for the udev db for the partitions even though though udev might
simplynot be around and via the DISSECT_IMAGE_NO_UDEV flag were
explicitly told not to bother.

Fixes: #19377