Doc/nixosOptionsDoc: fix baseModules cannot be empty (#430354)

  let

    # Evaluate a NixOS configuration

    eval = import (pkgs.path + "/nixos/lib/eval-config.nix") {

      # Overridden explicitly here, this would include all modules from NixOS otherwise.

      # See: docs of eval-config.nix for more details

      baseModules = [];

      modules = [

        ./module.nix

      ];