Creating a non-strict custom aggregate that initializes to the first value

Hi all,

I'm trying to create a custom aggregate function that returns the value
from the first row (possibly null).

For ex.

Table t
a | b
-----
1 | A
2 | NULL

SELECT my_first(b order by a) => A
SELECT my_first(b order by a DESC) => NULL

The straightforward way would seem to be something like

CREATE OR REPLACE FUNCTION public.first_agg ( anyelement, anyelement )
RETURNS anyelement LANGUAGE sql IMMUTABLE STRICT AS $$
SELECT $1;
$$;

but if that is declared strict then it would take the first non-null value
and return A in my second example, if declared non-strict then the initial
state would be fed as null rather then the first value. Is there a way to
declare the function non-strict (so that null values are passed) but still
have it initialize to the first value like it would if it was strict?

Thanks for the help,
Tim