Postgresql trigger check if NULL - postgresql

Question

I have a table where rows described by an unique ID are meant to be updated from an automatic process on a regular bassis.
INSERT INTO mytable(x,y,z) ON CONFLICT(x) DO UPDATE SET y=A...
USE CASE :
Some rows do not have a value yet, the next cycle might set some values.
Other rows already have values, but the next cycle might have NULL values to update. In this case, I don't want the NULL values to replace the current values.
I created this trigger :
CREATE OR REPLACE FUNCTION public.clean_update()
RETURNS trigger
LANGUAGE 'plpgsql'
COST 100
VOLATILE NOT LEAKPROOF
AS $BODY$
BEGIN
NEW.company = COALESCE(NEW.company,OLD.company);
NEW.first_name = COALESCE(NEW.first_name,OLD.first_name);
NEW.last_name = COALESCE(NEW.last_name,OLD.last_name);
NEW.address = COALESCE(NEW.address,OLD.address);
NEW.country = COALESCE(NEW.country,OLD.country);
NEW.phone = COALESCE(NEW.phone,OLD.phone);
NEW.mail = COALESCE(NEW.mail,OLD.mail);
NEW.evaluation = COALESCE(NEW.evaluation,OLD.evaluation);
NEW.positive = COALESCE(NEW.positive,OLD.positive);
NEW.neutral = COALESCE(NEW.neutral,OLD.neutral);
NEW.negative = COALESCE(NEW.negative,OLD.negative);
NEW.nb_products = COALESCE(NEW.nb_products,OLD.nb_products);
NEW.nb_sales = COALESCE(NEW.nb_sales,OLD.nb_sales);
NEW.nb_sale_url = COALESCE(NEW.nb_sale_url,OLD.nb_sale_url);
NEW.unregistered = COALESCE(NEW.unregistered,OLD.unregistered);
RETURN NEW;
END;
$BODY$;
With this trigger, the updated value is not replaced if its NEW.value is NULL, great.
The problem comes when updating only some of the columns :
ERROR: record "new" has no field "nb_sale_url"
Is there a way to check if NEW.x exists event if it's null ?
Or maybe there is a simplier way to do it