NEW YORK — The Knicks have a new center and, depending on the separate negotiations with Mitchell Robinson, he’ll probably be a backup. Isaiah Hartenstein, a 7-foot German, agreed to a two-year, $18 million deal, a source confirmed Thursday. The 24-year-old blossomed the last two seasons with the Cavaliers and Clippers, emerging in the NBA as a second-round pick who began in the G League. He averaged 8.3 points, 4.9 rebounds while shooting 63% in 17.9 minutes last season for the Clippers, who failed to make the playoffs after losing twice in the play-in tournament. Assuming Robinson re-signs a…