nixos/autosuspend: assert at least one check is configured

  };

  config = mkIf cfg.enable {

    assertions = [

      {

        assertion = cfg.checks != { };

        message = "`services.autosuspend.checks` must contain at least one activity check.";

      }

    ];

    systemd.services.autosuspend = {

      description = "A daemon to suspend your server in case of inactivity";

      documentation = [ "https://autosuspend.readthedocs.io/en/latest/systemd_integration.html" ];