OZG-4095 todo: Add nullable reminder

src/main/java/de/ozgcloud/nachrichten/postfach/osiv2/transfer/PostfachApiFacadeService.java

@@ -23,7 +23,7 @@ public class PostfachApiFacadeService {
 	private final RequestMapper requestMapper;
 	private final  ResponseMapper responseMapper;
 
-	private static int MAX_NUMBER_RECEIVED_MESSAGES = 100;
+	private static final int MAX_NUMBER_RECEIVED_MESSAGES = 100;
 
 	public void sendMessage(PostfachNachricht nachricht) {
 		messageExchangeApi.sendMessage(
@@ -34,6 +34,7 @@ public class PostfachApiFacadeService {
 
 	public Stream<PostfachNachricht> receiveMessages() {
 		var response = messageExchangeApi.receiveMessages(MAX_NUMBER_RECEIVED_MESSAGES, 0);
+		// TODO response.getMessages() is Nullable
 		return response.getMessages().stream().map(this::fetchMessageByGuid);
 	}