nixos/ssh: take care not to accept empty host key files

In case of a power loss shortly after first boot,
the host keys gernerated by ssh-keygen could exist
in the file system but have zero size, preventing
sshd from starting up.

This commit changes the behaviour to generate host
keys if the file either does not exist or has zero
size, fixing the problem on the next boot.

Thanks to @SuperSandro2000 for figuring this out.

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

@@ -439,7 +439,7 @@ in
                 mkdir -m 0755 -p /etc/ssh
 
                 ${flip concatMapStrings cfg.hostKeys (k: ''
-                  if ! [ -f "${k.path}" ]; then
+                  if ! [ -s "${k.path}" ]; then
                       ssh-keygen \
                         -t "${k.type}" \
                         ${if k ? bits then "-b ${toString k.bits}" else ""} \