Allow omitting cmdline when generating UKIs

Per systemd-stub(7),

If UEFI SecureBoot is enabled and the ".cmdline" section is present in the executed image, any attempts to override the kernel command line by passing one as invocation parameters to the EFI binary are ignored. Thus, in order to allow overriding the kernel command line, either disable UEFI SecureBoot, or don't include a kernel command line PE section in the kernel image file. If a command line is accepted via EFI invocation parameters to the EFI binary it is measured into TPM PCR 12 (if a TPM is present).

From what I see, mkinitcpio does not allow to omit embedding the kernel command line string, https://gitlab.archlinux.org/archlinux/mkinitcpio/mkinitcpio/-/blob/74573ffe1bee956925c9c1320210ad68dcff70fe/mkinitcpio#L343-358

==> WARNING: Note: /etc/kernel/cmdline does not exist and --cmdline is unset!
==> WARNING: Reusing current kernel cmdline from /proc/cmdline

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