Creating trigger with optional arg?

Is it possible to have a variable number of elements in TG_ARGV in a
trigger?

Arguments 1-3 are required, and I'd like a NULL check on TG_ARGV[4] as an
optional message, so it could be called in one of two ways:

create trigger tg_mytrig after insert on mytable for each row execute
procedure my_trigger('opt1', 'opt2', 'opt3');
create trigger tg_mytrig after insert on mytable for each row execute
procedure my_trigger('opt1', 'opt2', 'opt3', 'optional opt4');

With some kind of null check on TG_ARGV[4] inside the trigger.

--
Wells Oliver
wells(dot)oliver(at)gmail(dot)com <wellsoliver(at)gmail(dot)com>