Because `virtualisation.diskSize = null` does result in a broken vm runner,
see https://github.com/NixOS/nixpkgs/issues/292901.
diskSize was declared to be nullable when it first got types in a
tree-wide commit:
30f0faac22
But it seemingly never actually supported it, as "${cfg.diskSize}M" is
passed to qemu-img create, which doesn't allow an empty size parameter.
closes: https://github.com/NixOS/nixpkgs/issues/292901
53df1e4ece