Tongues wagged when fashionistas realized Marisa Berenson — grandaughter of designer Elsa Schiaparelli who was famously dubbed, “the girl of the ’70s” by Yves Saint Laurent — was absent from the star-studded Met Ball Monday celebrating the museum’s “Schiaparelli and Prada: Impossible Conversations” exhibition. “She was absolutely invited,” a source said, explaining there was no oversight in Vogue editor Anna Wintour’s impeccable guest list. Berenson solved the mystery by e-mailing us, “The fact is that I am in Africa since two months working on projects of mine that I cannot leave, but I am sure I will . . . see the exhibition when I am able to be back in NY.”