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.

src/kernel-install/kernel-install.in

@@ -158,7 +158,7 @@ if [ -z "$MACHINE_ID" ] && [ -f /etc/machine-info ]; then
     [ -n "$MACHINE_ID" ] && \
         log_verbose "machine-id $MACHINE_ID acquired from /etc/machine-info"
 fi
-if [ -z "$MACHINE_ID" ] && [ -f /etc/machine-id ]; then
+if [ -z "$MACHINE_ID" ] && [ -s /etc/machine-id ]; then
     read -r MACHINE_ID </etc/machine-id
     [ -n "$MACHINE_ID" ] && \
         log_verbose "machine-id $MACHINE_ID acquired from /etc/machine-id"