NEW YORK - Former pitchman John Nebel proved that Barnum was right last week when he presented double talk expert James Randi on his all-night jockey show over WOR here. Nebel, who recently replaced that controversial idol of the "night people" egghead Jean Shepherd, on the station's early a.m. seg, deliberately provoked an argument with Randi (introduced as Dr. Astro Keezlegar, a noted "Canadian metaphysician") on the air, claiming he couldn't understand him. Nebel was promptly swamped with calls from angry "night people" many of whom insisted they understood the doctor's double-talk routine completely. Meanwhile, Shepherd (who has a weekly Sunday night show on WOR now) cashed in on his front-page publicity this week by signing a contract to do a daily network show over ABC, starting October 22, from 10:05 to 11:55 p.m.