CHICAGO — A judge on Thursday acquitted three Chicago officers of trying to cover up the 2014 shooting of Laquan McDonald, dismissing as just one perspective the shocking dashcam video of the black teenager’s death that led to protests, a federal investigation of the police department and the rare murder conviction of an officer. In casting off the prosecution’s entire case, Judge Domenica Stephenson seemed to accept many of the same defense arguments that were rejected in October by jurors who convicted officer Jason Van Dyke of second-degree murder and aggravated battery.