nixos/ssh: remove empty host key files before generating new ones

In a previous PR [1], the conditional to generate a new host key file
was changed to also include the case when the file exists, but has zero
size. This could occur when the system is uncleanly powered off shortly
after first boot.

However, ssh-keygen prompts the user before overwriting a file. For
example:

$ touch hi
$ ssh-keygen -f hi
Generating public/private rsa key pair.
hi already exists.
Overwrite (y/n)?

So, lets just try to remove the empty file (if it exists) before running
ssh-keygen.

[1] https://github.com/NixOS/nixpkgs/pull/141258

nixos/modules/services/networking/ssh/sshd.nix

@@ -441,6 +441,7 @@ in
 
                 ${flip concatMapStrings cfg.hostKeys (k: ''
                   if ! [ -s "${k.path}" ]; then
+                      rm -f "${k.path}"
                       ssh-keygen \
                         -t "${k.type}" \
                         ${if k ? bits then "-b ${toString k.bits}" else ""} \