His special night was nothing but net. A brave Manhattan boy who has “kept smiling” through years of excruciating sickle cell disease had the time of his life at Monday’s Knicks game thanks to a charity that partners with Madison Square Garden, his mom told The Post. Marvell CharlesPierre Jr., 8, who goes by “MJ,” scored a free ticket to Game 3 of the NBA Finals Monday as part of the Garden of Dreams program — and was “hyped up” with “joy,” said his mother, Angelina Rosado. “Knowing how much