Prevent unnecessary config values

lib/modules/programs.nix

@@ -5,17 +5,20 @@ let
 
   mkUsersOption = programs: osConfig: mkOption {
     type = types.attrsOf (types.submodule (
-      { ... }: {
+      { config, ... }: {
         options = {
           inherit programs;
         };
 
         config = {
-          programs = builtins.mapAttrs
+          programs = lib.attrsets.concatMapAttrs
             (
-              name: config: {
-                enable = mkDefault config.enable;
-              }
+              name: program:
+                if (builtins.elem name (builtins.attrNames config.programs)) then {
+                  ${name} = {
+                    enable = mkDefault program.enable;
+                  };
+                } else { }
             )
             osConfig.programs;
         };