ec2-data: ensure providing a SSH host key is actually optional

27016659046a8f8e7b4fd61ecbceaf9f5e306258 broke this.

nixos/modules/virtualisation/ec2-data.nix

@@ -43,6 +43,7 @@ with lib;
 
             mkdir -m 0755 -p /etc/ssh
 
+            if [ -s "$userData" ]; then
               key="$(sed 's/|/\n/g; s/SSH_HOST_DSA_KEY://; t; d' $userData)"
               key_pub="$(sed 's/SSH_HOST_DSA_KEY_PUB://; t; d' $userData)"
               if [ -n "$key" -a -n "$key_pub" -a ! -e /etc/ssh/ssh_host_dsa_key ]; then
@@ -56,6 +57,7 @@ with lib;
                   (umask 077; echo "$key" > /etc/ssh/ssh_host_ed25519_key)
                   echo "$key_pub" > /etc/ssh/ssh_host_ed25519_key.pub
               fi
+            fi
           '';
 
         serviceConfig.Type = "oneshot";