nixos/display-managers: fix assertion

            lib.assertMsg (d != null -> (lib.types.str.check d && lib.elem d config.services.displayManager.sessionData.sessionNames)) ''

                Default graphical session, '${d}', not found.

                Valid names for 'services.displayManager.defaultSession' are:

                  ${lib.concatStringsSep "\n  " cfg.displayManager.sessionData.sessionNames}

                  ${lib.concatStringsSep "\n  " cfg.sessionData.sessionNames}

              '';

        };

        default = null;