NEW YORK AND PHILADELPHIA — Something sad, even tragic, haunts the work of Jasper Johns. Now 91, the American artist is the subject of a mammoth joint retrospective, at the Whitney Museum of American Art in New York and the Philadelphia Museum of Art, including more than 500 works from almost seven decades of artistic production. This is the first genuine, must-see blockbuster exhibition to open since the pandemic began in 2019, and it might be an occasion for euphoria. No living American artist has been so productive and so influential for so long. But for all its brilliance, the work of Johns is bleak. Almost everything you want from art is present: cogent ideas, flawless execution, a sweeping sense of history and a personal stamp that identifies every piece as unmistakably the work of a unique and capacious mind. But where are love and generosity?