setup: show "n/a" for empty boot option labels
setup: show "n/a" for empty boot option labels

miguelinux/gummiboot2systemd-boot,todorez/tummiboot,aaronp24/gummiboot,aaronp24/gummiboot,karelzak/gummiboot,karelzak/gummiboot,todorez/gummiboot-multiboot2,FrozenCow/gummiboot,liliang365/my_gummiboot,trilean/gummiboot,todorez/tummiboot,todorez/gummiboot-multiboot2,miguelinux/gummiboot2systemd-boot,FrozenCow/gummiboot,liliang365/my_gummiboot,freedesktop-unofficial-mirror/gummiboot,trilean/gummiboot,freedesktop-unofficial-mirror/gummiboot