Incompatible with root in btrfs subvolume

[aedalzotto]

When creating an entry for a root installed in a btrfs subvolume, the entry stays with empty root and doesn't add the subvol argument:

lsblk: /dev/nvme0n1p2[/@root]: not a block device
lsblk: /dev/nvme0n1p2[/@root]: not a block device