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.

Changed files
+6
nixos
modules
services
+6
nixos/modules/services/desktops/profile-sync-daemon.nix
···
};
config = mkIf cfg.enable {
+
assertions = [
+
{ assertion = cfg.users != [];
+
message = "services.psd.users must contain at least one user";
+
}
+
];
+
systemd = {
services = {
psd = {