OZG-4094 Fix NPE for empty messages

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

 package de.ozgcloud.nachrichten.postfach.osiv2.transfer;
 
+import java.util.Collection;
+import java.util.Optional;
 import java.util.UUID;
 import java.util.stream.Stream;
 
@@ -34,7 +36,10 @@ public class PostfachApiFacadeService {
 
 	public Stream<PostfachNachricht> receiveMessages() {
 		var response = messageExchangeApi.receiveMessages(MAX_NUMBER_RECEIVED_MESSAGES, 0);
-		return response.getMessages().stream().map(this::fetchMessageByGuid);
+		return Optional.ofNullable(response.getMessages())
+				.stream()
+				.flatMap(Collection::stream)
+				.map(this::fetchMessageByGuid);
 	}
 
 	PostfachNachricht fetchMessageByGuid(final MessageExchangeReceiveMessage message) {