analyzer `cp_diff`: score is `None` => Causes Error

[pcattori]

Score is None when a checkmate is detected.

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{
  'pv': {1: [Move.from_uci('d7c7'), Move.from_uci('d5e6'), Move.from_uci('c7d8'), Move.from_uci('e6d6'), Move.from_uci('d8e8'), Move.from_uci('g4g5'), Move.from_uci('e8f7'), Move.from_uci('d6f6'), Move.from_uci('f7e8'), Move.from_uci('f6e6'), Move.from_uci('e8d8'), Move.from_uci('e6d6'), Move.from_uci('d8c8'), Move.from_uci('d6e7'), Move.from_uci('b4b5'), Move.from_uci('e7c5'), Move.from_uci('c8d7'), Move.from_uci('c5b5'), Move.from_uci('d7e6'), Move.from_uci('b5c5'), Move.from_uci('e6d7'), Move.from_uci('g5f6'), Move.from_uci('d7d8'), Move.from_uci('c5e7'), Move.from_uci('d8c8'), Move.from_uci('e7b4'), Move.from_uci('c8d7')]},
  'depth': 21,
  'seldepth': 38,
  'multipv': 1,
  'currline': {},
  'score': {1: Score(cp=None, mate=-18)},
  'tbhits': 0,
  'nodes': 366629,
  'refutation': {},
  'nps': 3629990,
  'time': 101
}

[pcattori]

now NotImplementedError raised instead