mdformat: don't apply empty `plugins` option

[landure]

Describe the bug

In mdformat module, default for plugins option is _: [].
Its applied with programs.mdformat.finalPackage = cfg.package.withPlugins cfg.plugins;

This override a mdformat package with withPlugins already defined.

To Reproduce

Steps to reproduce the behavior:

    treefmt = {
      enable = mkDefault true;

      config.programs.mdformat = {
        enable = true;
        package = pkgs.mdformat.withPlugins (plugins: with plugins; [mdformat-frontmatter]);
      };
    };

treefmt's mdformat doesn't have frontmatter plugin.

Expected behavior

plugins option default to null, and isn't applied if null, rather than applied with an empty plugins list.

Additional context

This is an issue when trying to have the same mdformat command behavior with git hooks and shell.