[unified_kernel_install] Really pick a default prefix name
[unified_kernel_install] Really pick a default prefix name

with empty string for args.image_id we end up with an odd looking
BOOT_BINARY which results in calling dracut as such:

dracut: Executing: /usr/bin/dracut --uefi --kver 5.11.17-gentoo-dist
--kernel-image /usr/src/linux-5.11.17-gentoo-dist/arch/x86/boot/bzImage
--kernel-cmdline "rhgb selinux=0 audit=0 rw" --force
/efi/EFI/Linux/-5.11.17-gentoo-dist.efi

-n test's for _none_ empty string; -z for an empty one.

systemd/mkosi,systemd/mkosi