Fix password hash file generation behavior

- Move the "create password hash file from hashed password" behavior to
  a separate variable, since having it in the default field of config
  would always cause the warning to trigger
- Change type of hashedPassword to `nullOr str`

mail-server/postfix.nix

@@ -16,7 +16,7 @@
 
 { config, pkgs, lib, ... }:
 
-with (import ./common.nix { inherit config lib; });
+with (import ./common.nix { inherit config pkgs lib; });
 
 let
   inherit (lib.strings) concatStringsSep;