Barbra Streisand, the Broadway balladeer and pop craftswoman, seemed shocked that she was the one to deliver the Academy's first-ever Best Original Song award to a hardcore rap tune, Eminem's ''Lose Yourself'' from ''8 Mile.''  Fortunately for Babs, Slim didn't show. It seemed so empty without him, but co-composer Luis Resto accepted the trophy, wearing a blazer over a basketball jersey that could also have earned him the Most Casually Dressed Award.