Fix reading /etc/machine-id in kernel-install (#25388)

Fix reading /etc/machine-id in kernel-install (#25388)

* Fix reading /etc/machine-id in kernel-install

The kernel-install script has code to read the contents of
/etc/machine-id into the MACHINE_ID variable. Depending
on the variable content kernel-install either logs the
value or creates a new machine id via 'systemd-id128 new'.
In that logic there is one issue. If the file /etc/machine-id
exists but is empty, the script tries to call read on an
empty file which return with an exit code != 0. As the
script code also uses 'set -e', kernel-install will exit at
this point which is unexpected.

The condition of an empty /etc/machine-id file exists for
example when building OS images, which should initialize the
system id on first boot but not staticly inside of the image.
afaik an empty /etc/machine-id is also a common approach
to make systemd indicate that it should create a new system
id. Because of this, the commit makes sure the reading of
/etc/machine-id does not fail in any case such that the
handling of the MACHINE_ID variable takes place.