nixos: Allow empty hostnames again

This fixes a regression from 993baa58 which requires
networking.hostName to be a valid DNS label [0].
Unfortunately we missed the fact that the hostnames may also be empty,
if the user wants to obtain it from a DHCP server. This is even required
by a few modules/images (e.g. Amazon EC2, Azure, and Google Compute).

[0]: https://github.com/NixOS/nixpkgs/pull/76542#issuecomment-638138666