What group on the island of Muckle Green Holm has a population of 0?
CREATE TABLE "main_list" (
  "island" text,
  "group" text,
  "area_ha" text,
  "population" text,
  "height_m" real
);
SELECT "group" FROM "main_list" WHERE "population"='0' AND "island"='muckle green holm';