postgresql – How to allow NULL in foreign keys of a compound primary key

I want to have NULLs in foreign keys of a compound primary key. This is an example of what I would expect to be valid data.

I cannot figure out why the following SQL in postgres gives NOT NULL violation constraint me when inserting NULL as variant_id.

CREATE TABLE IF NOT EXISTS inventory.price (
  product_id             UUID NOT NULL, -- this has to be always to a valid product
  variant_id             UUID,          -- this could be NULL
  amount                 MONEY NOT NULL,
  created_at             TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT now(),
  -- Constraints
  CONSTRAINT inventory_price_pkey PRIMARY KEY (product_id, variant_id),
  CONSTRAINT inventory_price_inventory_product_fkey FOREIGN KEY (product_id)
    REFERENCES inventory.product (id) MATCH FULL,
  CONSTRAINT inventory_price_inventory_variant_fkey FOREIGN KEY (variant_id)
    REFERENCES inventory.variant (id) MATCH SIMPLE,
  CONSTRAINT inventory_price_amount_gt_0 CHECK (amount > '0'::money)
);

And the inspection to information_schema confirms the non-nullable constraint.