Make darwin/home/nixosModules not be set when no modules are present

Right now, if you use blueprint in a flake, the outputs
darwin/home/nixosModules will be set to a empty attribute if there are
no modules, this can be easily fixed with `lib.mkIf`. This error also
exists with darwin/system/nixosConfigurations, and using `lib.mkIf` will
make those lines quite long, I don't have a solution for that currently.

Author: Your Name

lib/default.nix

@@ -439,6 +439,9 @@ let
         );
     in
     # FIXME: maybe there are two layers to this. The blueprint, and then the mapping to flake outputs.
+    lib.mkIf (modules ? nixos) {nixosModules = modules.nixos or {};} //
+    lib.mkIf (modules ? home) {homeModules = modules.home or {};} //
+    lib.mkIf (modules ? darwin) {darwinModules = modules.darwin or {};} //
     {
       formatter = eachSystem (
         { pkgs, perSystem, ... }:
@@ -590,11 +593,6 @@ let
 
       inherit modules;
 
-      darwinModules = modules.darwin or { };
-      homeModules = modules.home or { };
-      # TODO: how to extract NixOS tests?
-      nixosModules = modules.nixos or { };
-
       templates = importDir (src + "/templates") (
         entries:
         lib.mapAttrs (