NEW YORK — Donald Kagan had been in prison for murder for more than a decade when doubts about his guilt arose from an uncommon source: the former judge who had convicted him. The reason was more extraordinary still: The now-retired, white jurist felt he had been swayed by bias against Kagan, also white, in the killing of a black man. On Wednesday, another judge upheld the conviction in the 1998 case. Acting Brooklyn state Supreme Court Justice ShawnDya Simpson said she found the case emotionally wrenching …