public Query buildQuery(final ExpressionOperator expression) {
        Query query = null;
        if (expression != null) {
            query = getQuery(expression);
        }
        if (query == null) {
            query = MatchAllQuery.of(q -> q)._toQuery();
        }
        return query;
    }
Convert our query objects to an Elasticsearch Query DSL query.
stroom-search/stroom-search-elastic/src/main/java/stroom/search/elastic/search/SearchExpressionQueryBuilder.java