fix null pointer

Author: pidoubleyou

src/main/java/de/mediathekview/mserver/crawler/zdf/tasks/ZdfFilmTask.java

@@ -108,7 +108,7 @@ private static void updateTitle(final String aLanguage, final Film aFilm) {
   @Override
   protected void processRestTarget(ZdfFilmDto aDTO, WebTarget aTarget) {
     final Optional<DownloadDto> downloadDto =
-        deserialize(aTarget, OPTIONAL_DOWNLOAD_DTO_TYPE_TOKEN);
+        deserializeOptional(aTarget, OPTIONAL_DOWNLOAD_DTO_TYPE_TOKEN);
     if (downloadDto.isPresent()) {
       try {
         addFilm(downloadDto.get(), createFilm(aDTO, downloadDto.get()), aDTO.getVideoType());