BOSTON — Gov. Deval Patrick canceled a speech in Plymouth on Thursday after learning that union members, mostly off-duty police officers, were preparing to greet him with a picket line. About 250 people from the South Shore and elsewhere in the state were waiting outside the Radisson Conference Center, said Jim Barry, spokesman for the Boston Police Patrolmen’s Association. Patrick was set to speak at an annual AFL-CIO conference. Before his arrival, the labor group’s executive committee voted to join the police officers outside, Barry said, so Patrick would have encountered not just a picket line, but also an empty...