Donald Trump’s niece says that she’s able to predict the ex-president’s next “trigger” because she knows him so well. Specifically, Mary Trump was referring to reports that, recently, the former president’s rallies have lacked the vigor that they had in the past. She also pointed to Trump reportedly having “empty seats all over the arena.” “Donald isn’t going to like this,” Mary Trump said Wednesday. “As he tries to grapple with Nikki Haley’s stronger-than-expected showing in New Hampshire, Donald’s campaign has a new problem: shrinking crowd sizes.” Want more breaking political news? Click he…