psd service: assert that at least one user must be configured

Using the default config, a user will experience a run-time failure.
This is poor UX, assert the requirement up-front.

nixos/modules/services/desktops/profile-sync-daemon.nix

@@ -86,6 +86,12 @@ in {
   };
 
   config = mkIf cfg.enable {
+    assertions = [
+      { assertion = cfg.users != [];
+        message = "services.psd.users must contain at least one user";
+      }
+    ];
+
     systemd = {
       services = {
         psd = {