Changed `services.zerotierone.localConf` default value to `null` and type to `nullOr attrs` per input.
Changed `systemd` preStart script to delete
`/var/lib/zerotier-one/local.conf` when it is a symlink, and rename when it is an actual file, then only create a symlink to the nix store when `services.zerotierone.localConf` is not null.