NEW YORK — A rider was stabbed in the back during an unprovoked attack at a Harlem subway station early Friday, police said. The early-morning attack was the latest in a series of violent encounters in city subways, cops said. The 43-year-old victim was waiting at the A/B/C/D platform at the W. 125th St. subway station near St. Nicholas Ave. about 4:30 a.m. when a man in his 20s stormed up to him and attacked, cops said. The suspect stabbed the commuter once in the back and ran off. EMS rushed the victim to Mount Sinai Morningside Hospital with a minor wound. His attacker, who was wearing a bl…