Fix GameNode.set_eval() dropping mate 0 scores

[kwy404]

Root cause: set_eval() only writes a mate annotation when score.white().mate() is truthy. For a mated position the score is Mate(0) (or MateGiven from the other side), so mate() returns 0, the branch is skipped and nothing is written. Worse, _replace_or_add_annotation("") then removes an existing [%eval ...] from the comment. Since e32f9e1, GameNode.eval() parses [%eval #0], so the reader and writer disagree:

game = chess.pgn.read_game(io.StringIO("1. f3 e5 2. g4 Qh4# { [%eval #0] } 0-1"))
node = game.end()
node.set_eval(node.eval())
node.comments  # [] instead of ["[%eval #0]"]

The same happens when annotating a game with an engine: for the final checkmate position the engine reports score mate 0, and node.set_eval(info["score"]) writes nothing.

Fix: check mate() is not None instead of truthiness, so Mate(0) and MateGiven are written as [%eval #0], which eval() already reads back as the side to move being mated.

Test: test_eval_mate_zero reads the PGN above, writes the parsed eval back and checks that the comment is still [%eval #0] and that eval() still returns Mate(0). It fails on master with Lists differ: [] != ['[%eval #0]'] and passes with the fix. The full python test.py suite passes.