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

@@ -590,10 +590,10 @@ let
 
       inherit modules;
 
-      darwinModules = modules.darwin or { };
-      homeModules = modules.home or { };
+      darwinModules = lib.mkIf (modules.darwin or null != null) (modules.darwin {});
+      homeModules = lib.mkIf (modules.home or null != null) (modules.home or {});
       # TODO: how to extract NixOS tests?
-      nixosModules = modules.nixos or { };
+      nixosModules = lib.mkIf (modules.nixos or null != null) (modules.nixos or {});
 
       templates = importDir (src + "/templates") (
         entries: