fix ClassCastException if contentOwner missing

Author: pidoubleyou

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

@@ -29,8 +29,10 @@ public ZdfPubFormResult deserialize(
 
     for (JsonElement element : nodes) {
       JsonObject node = element.getAsJsonObject();
-      final Optional<String> sender =
-          JsonUtils.getElementValueAsString(node.getAsJsonObject("contentOwner"), "title");
+      final Optional<String> sender = Optional.empty();
+      if (!node.get("contentOwner").isJsonNull()) {
+        JsonUtils.getElementValueAsString(node.getAsJsonObject("contentOwner"), "title");
+      }  
       final Optional<String> topic = JsonUtils.getElementValueAsString(node, "title");
       final Optional<String> countSeasons = JsonUtils.getElementValueAsString(node, "countSeasons");
       if (ZdfConstants.PARTNER_TO_SENDER.containsKey(sender.orElse("ZDF"))) {