private Tuple2<Integer, Reply> createEmptyReplyResult() {
        return new Tuple2<>(null, new EmptyReply());
    }
Encapsulated logic for merging replies from 1-n related DocumentProtocol messages.
For internal use only. Not multithread safe.
documentapi/src/main/java/com/yahoo/documentapi/messagebus/protocol/ReplyMerger.java