Verify parent_dev before calling udev_device_get_sysattr_value

Not all udev devices have parent devices.
Calling udev_device_get_ functions yield an assertion error
if called with a NULL pointer.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Sietse <sietse@wizdom.nu>
Co-authored-by: Sietse <sietse@wizdom.nu>
Closes #16705
Closes #16717