@Test
    void testEmptyExpressionTree() {
        // AND (op1)
        final ExpressionOperator.Builder op1 = ExpressionOperator.builder();
        FilterTermsTreeBuilder.convertExpresionItemsTree(op1.build());
    }
Verify that a tree of {@link stroom.query.api.ExpressionItem} objects can be converted
correctly into a {@link FilterTermsTree}
stroom-statistics/stroom-statistics-impl-sql/src/test/java/stroom/statistics/impl/sql/search/TestFilterTermsTreeBuilder.java