What company constructed the vehicle when the driver shows as not held?
CREATE TABLE "winners_of_the_moroccan_grand_prix" (
  "year" text,
  "driver" text,
  "constructor" text,
  "category" text,
  "location" text,
  "report" text
);
SELECT "constructor" FROM "winners_of_the_moroccan_grand_prix" WHERE "driver"='not held';