How to check if an array is empty in Postgres

I have a postgres storage procedure:

CREATE OR REPLACE FUNCTION get_stats(
    _start_date timestamp with time zone,
    _stop_date timestamp with time zone,
    id_clients integer[],
    OUT date timestamp with time zone,
    OUT profit,
    OUT cost
)
RETURNS SETOF record
LANGUAGE plpgsql
AS $$
DECLARE
    query varchar := '';
BEGIN
... --lof of code
IF id_clients IS NOT NULL THEN
    query := query||' AND id = ANY ('||quote_nullable(id_clients)||')';
END IF;
... --other code
END;
$$;

So if I run query something like this:

SELECT * FROM get_stats('2014-07-01 00:00:00Etc/GMT-3', '2014-08-06 23:59:59Etc/GMT-3', '{}');

Generated query has this condition:

"... AND id = ANY('{}')..."

But if an array is empty this condition should not be represented in query.

How can I check if the array of clients is not empty? Thanks.

UPD: Also I've tried two variants:

IF ARRAY_UPPER(id_clients) IS NOT NULL THEN
    query := query||' AND id = ANY ('||quote_nullable(id_clients)||')';
END IF;

And

IF ARRAY_LENGTH(id_clients) THEN
    query := query||' AND id = ANY ('||quote_nullable(id_clients)||')';
END IF;

In both cases I got this error: ARRAY_UPPER(ARRAY_LENGTH) doesn't exists ;