handle null elements

Author: pidoubleyou

src/main/java/de/mediathekview/mserver/crawler/zdf/json/ZdfLetterPageDeserializer.java

@@ -30,6 +30,9 @@ public PagedElementListDTO<ZdfTopicUrlDto> deserialize(
     JsonArray nodes = content.getAsJsonArray("nodes");
 
     for (JsonElement element : nodes) {
+      if (element == null || element.isJsonNull()) {
+        continue;
+      }
       JsonObject node = element.getAsJsonObject();
       final Optional<String> sender =
           JsonUtils.getElementValueAsString(node.getAsJsonObject("contentOwner"), "title");