@Override
        boolean serialize(StringBuilder destination, NullItem item) {
            throw new NullItemException("NullItem encountered in query tree. This is usually a symptom of an invalid " +
                                        "query or an error in a query transformer.");
        }
Serialize Vespa query trees to YQL+ strings.

@author Steinar Knutsen
container-search/src/main/java/com/yahoo/search/yql/VespaSerializer.java