Converting empty input strings to Nulls

Applications accessing my PostgreSQL 8.0 database like to submit no-value
date column values as empty strings rather than as Nulls.  This, of course,
causes this PG error:

SQL State: 22007
ERROR: invalid input syntax for type date: ""

I'm looking for a way to trap this bad input at the database level, quietly
convert the input empty strings to Null, and store the Null in the date
column.  I tried a BEFORE INSERT OR UPDATE trigger evoking this function ...

CREATE OR REPLACE FUNCTION "public"."empty_string_to_null"()
RETURNS trigger AS
$BODY$
BEGIN
    IF CAST(NEW.birth_date AS text) = '' THEN
        NEW.birth_date = Null;
    END IF;
    RETURN NEW;
END;
$BODY$
LANGUAGE 'plpgsql' VOLATILE;

... but an empty string still evokes the error even before this function is
triggered.

Is there a way to convert empty strings to Nulls before the error is evoked?

~ TIA
~ Ken