LP:702345 - Empty query with SUM and HAVING, no GROUP BY returns NULL instead of empty result.

The following test case (for LP BUG#609121) extracted from subselect_mat.test:

create table t1 (c1 int);
create table t2 (c2 int);
insert into t1 values (1);
insert into t2 values (2);

SELECT SUM(c1) c1_sum FROM t1 WHERE c1 IN (SELECT c2 FROM t2) HAVING c1_sum;

incorrectly produces a NULL instead of an empty result.