As promised , Oscar de la Renta really did sing with Barbara Walters — oh, and Bette Midler and Sarah Jessica Parker — in his Madison Avenue store for Fashion’s Night Out. SJP, who arrived a bit late, straight from the SATC 2 set, insisted she was pressed into the chorale unwittingly. “I didn’t know there was going to be singing here, period. So I certainly didn’t know that Mr. de la Renta could sing,” Parker told us after the performance. Walters said she and De la Renta had planned a duet, a