checking for NULLS in aggregate

Hello,

I've defined a function to calculate standard deviation of angular
values:

CREATE AGGREGATE public.stddev(angle_vectors) (
  SFUNC=array_append,
  STYPE=angle_vectors[],
  FINALFUNC=angle_vectors_stddev_yamartino
);

The type angle_vectors is simply an array:

CREATE TYPE public.angle_vectors AS
   (x double precision,
    y double precision);
COMMENT ON TYPE public.angle_vectors
  IS 'This type holds the x (sine) and y (cosine) components of angle(s).';

How can I protect this function so that NULL is returned whenever *all*
input rows are NULL?

Thanks for any feedback,

--
Seb