per-user module: create profiles only when needed

Refs https://github.com/NixOS/nixpkgs/pull/25712

krebs/3modules/per-user.nix

@@ -15,16 +15,19 @@ in {
   };
   config = {
     environment = {
-      etc = flip mapAttrs' cfg (name: { packages, ... }: {
+      etc =
+        mapAttrs'
+          (name: per-user: {
             name = "per-user/${name}";
             value.source = pkgs.buildEnv {
               name = "per-user.${name}";
-          paths = packages;
+              paths = per-user.packages;
               pathsToLink = [
                 "/bin"
               ];
             };
-      });
+          })
+          (filterAttrs (_: per-user: per-user.packages != []) cfg);
       profiles = ["/etc/per-user/$LOGNAME"];
     };
   };