GREATEST() and LEAST() malfunction for NULL

Hi everyone,

On the latest trunk version (0308de9), GREATEST() and LEAST() seem to malfunction for NULL values:

CREATE TABLE t0(c0 INT);

INSERT INTO t0 VALUES (1);

SELECT * FROM t0 WHERE GREATEST(c0, NULL); -- unexpected: row is fetched

I would not expect the row to be fetched, because the WHERE condition should evaluate to NULL for that row:

SELECT GREATEST(c0, NULL) FROM t0; -- NULL