ON THE AIR TODAY: ZaSu Pitts , playing the role of Aunt Mamie in the CBS serial, Big Sister , at 11:30 A.M., E.S.T. (rebroadcast at 11:00 A.M., Pacific Time), sponsored by Rinso. That wistful little lady in Columbia’s Studio Four, eyeing the microphone so distrustfully, is ZaSu Pitts, who Eric Von Stroheim always insisted was the finest dramatic actress in America. She has been in the movies since 1917, and never has appeared on the screen, even in a small role, without bringing a delighted murmur from audiences. She is utterly without temperament or stuffiness, and has a heart as big as Radio City. She hasn’t given up movie work for radio , by any means. But she isn’t under contract to any one studio in Hollywood , and when the opportunity came along to spend a few months in New York and act in Big Sister it sort of appealed to her, she says. Asked if she has any movie plans for the future, ZaSu doesn’t commit herself. “I’m hoping,” she says. “I’ve been hoping for twent