nixos: Allow empty hostnames again

This fixes a regression from 993baa587c 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

nixos/modules/tasks/network-interfaces.nix

@@ -381,7 +381,7 @@ in
       # syntax). Note: We also allow underscores for compatibility/legacy
       # reasons (as undocumented feature):
       type = types.strMatching
-        "^[[:alpha:]]([[:alnum:]_-]{0,61}[[:alnum:]])?$";
+        "^$|^[[:alpha:]]([[:alnum:]_-]{0,61}[[:alnum:]])?$";
       description = ''
         The name of the machine. Leave it empty if you want to obtain it from a
         DHCP server (if using DHCP). The hostname must be a valid DNS label (see