how to deteck empty tables in outer joins

Im trying to do an outerjoin of two tables. The second one might be
empty. Normally I would use a query like:

CREATE TABLE a ( id INTEGER );
CREATE TABLE b ( id INTEGER );

SELECT * FROM a,b
WHERE a.id=b.id
UNION ALL
SELECT * FROM a,b
WHERE a.id NOT IN (b.id)
;

If the seconf table is empty the result is null, because the cartesian
product of table and null is null. What I want is to include a condition
that if b has no rows then just add null for the value of b ie.
SELECT *,NULL FROM a; How can I implement this?

- Einar Karttunen