nixos/firefox: fix eval with empty overrides (#464988)

    environment.systemPackages = [

      (cfg.package.override (old: {

        extraPrefsFiles =

          old.extraPrefsFiles

          (old.extraPrefsFiles or [ ])

          ++ cfg.autoConfigFiles

          ++ [ (pkgs.writeText "firefox-autoconfig.js" cfg.autoConfig) ];

        nativeMessagingHosts = old.nativeMessagingHosts ++ cfg.nativeMessagingHosts.packages;

        cfg = old.cfg // cfg.wrapperConfig;

        nativeMessagingHosts = (old.nativeMessagingHosts or [ ]) ++ cfg.nativeMessagingHosts.packages;

        cfg = (old.cfg or { }) // cfg.wrapperConfig;

      }))

    ];