Last week, Nia Long got the tap on the shoulder she’s been waiting for forever. It was a summons from Katherine Jackson , the matriarch of the Jackson family, whom she portrays in the new biopic, “ Michael .” Long steeled herself as she walked into a green room backstage at the Dolby Theatre and, when they came face to face, she froze for a moment. Then, she kneeled down, held the 95-year-old woman’s hand and said, “Thank you so much for all your sacrifices, for your love, for your patience, for your commitment to family.” “Mother Jackson,” as Long calls her, simply gazed back at her, so she continued: “I said, ‘Do you realize none of this would be possible without you? You are the beginning of the Jackson legacy.’ And she teared up. It was a beautiful moment, and it was confirmation for me that she’s very pleased with the film.…