An empty `/etc/machine-id` file does _not_ trigger systemd's "first
boot" logic. This means with `ConditionFirstBoot=true` will not run,
and the unit preset policy will not be applied. To ensure a first boot
behaves the way we want, we need to pre-populate the `/etc/machine-id`
file with the string `uninitialized\n`, per _machine-id(5)_ § _First
Boot Semantics_.