Make sure we resign when engine reports we are checkmated

Make sure we resign when engine reports we are checkmated

When a UCI engine is set thinking in a checkmated position, the PV
necessarily is empty, and instead of printing an empty 'pv' field in
as pv info, it might omit it alltogether. So we even accept a 'mate 0'
score when there is no PV, so that a following 'bestmove 0000' will
be interpreted as a loss (sending 'resign'), rather than a draw
(sending 1/2-1/2).