submodule-support: add default values for top-level configs

[name-snrl]

This way the end user can easily check whether the home-manager configuration is part of the module or not. Example of use:

{ lib, nixosConfig, ... }:
let
  mkIfNixos = lib.mkIf nixosConfig != null;
in
{
  programs.foot.enable = mkIfNixos true;
}

related: https://github.com/NixOS/nixpkgs/issues/311709 many thanks to @roberth for the explanation

Description

Checklist

• [x] Change is backwards compatible.

• [x] Code formatted with ./format.

• [x] Code tested through nix-shell --pure tests -A run.all or nix develop --ignore-environment .#all using Flakes.

• [ ] Test cases updated/added. See example.

• [x] Commit messages are formatted like

  {component}: {description}
  
  {long description}

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • [ ] Added myself as module maintainer. See example.

Maintainer CC

[name-snrl]

cc @rycee

[name-snrl]

@emilazy @rycee @SuperSandro2000 @roberth I apologize for mentioning again those who have already been mentioned before, but perhaps during the weekend I have a better chance to draw your attention to this simple but useful PR