What is the total number of Viewers with a Rank (Night) of n/a, and a Timeslot of 8:30 p.m.?
CREATE TABLE "weekly_ratings" (
  "episode" text,
  "air_date" text,
  "timeslot" text,
  "rating" real,
  "share" real,
  "18_49_rating_share" text,
  "viewers_m" real,
  "rank_timeslot" real,
  "rank_night" text,
  "rank_overall" real
);
SELECT COUNT("viewers_m") FROM "weekly_ratings" WHERE "rank_night"='n/a' AND "timeslot"='8:30 p.m.';