Kate Jackson's string of hit TV series--"The Rookies," "Charlie's Angels," "Scarecrow and Mrs. King"--is in danger of breaking. NBC says it is putting her faltering new effort, "Baby Boom," on hiatus. The network promises that the comedy will get another chance, however, after some unspecified "creative changes" are made. Meantime, NBC has given season pickups to three other new series--"Empty Nest," "Dear John" and the recently controversial "Midnight Caller"--and to two second-season shows, "Day by Day" and "In the Heat of the Night." NBC said that it will make an announcement next week on the future of "Tattinger's," the network's classy but low-rated drama series set in a New York eatery.