Tweak null move pruning conditions
Allow null move pruning only if last stat score < 30000.
Passed STC:
LLR: 2.95 (-2.94,2.94) [0.00,5.00]
Total: 12653 W: 2641 L: 2454 D: 7558
http://tests.stockfishchess.org/tests/view/5af1d8830ebc5968e6523edb
Passed LTC:
LLR: 2.97 (-2.94,2.94) [0.00,5.00]
Total: 19782 W: 3072 L: 2878 D: 13832
http://tests.stockfishchess.org/tests/view/5af1ee8c0ebc5968e6523ee4
Ideas for further work:
• Tune the stat score threshold.
• Try depth based stat score threshold.
• Try stat score condition for other prunings.
Closes https://github.com/official-stockfish/Stockfish/pull/1589
Bench: 5088156