Stockfish dev-20241222-f656fdfa

Simplify Zobrist keys for captures

The Zobrist keys for NO_PIECE are 0 so no need to special case captures.
Also the TranspositionTable reference passed to do_null_move() can be
const.

STC Simplification:
https://tests.stockfishchess.org/tests/view/6764a79a86d5ee47d9544005
LLR: 2.96 (-2.94,2.94) <-1.75,0.25>
Total: 114240 W: 29654 L: 29523 D: 55063
Ptnml(0-2): 329, 12360, 31620, 12473, 338

closes https://github.com/official-stockfish/Stockfish/pull/5731

No functional change