how to return 0 rows in function

Hi, I have simplified function like this:

----------------
CREATE OR REPLACE FUNCTION f(varchar) RETURNS varchar AS $FUNC$
DECLARE
addr ALIAS FOR $1;
BEGIN
-- return NULL;
-- return '';
END
$FUNC$ LANGUAGE 'plpgsql';
-----------------

This function is returning varchar and it always returns at least one row. How
can I make it to return 0 rows? Is it possible or not and how?
Returning NULL does not help (return NULL string in 1 row). Not to return a
value leads to error output.

Many thanks for help

Tomas