Add improving to a condition of NMP
This patch makes one of the conditions for null-move pruning depend on whether we're improving. Keep in mind that it relies on the "classical" definiton, rather than the refined one.
Passed STC: https://tests.stockfishchess.org/tests/view/678d5267d63764e34db49720
LLR: 2.94 (-2.94,2.94) <0.00,2.00>
Total: 38976 W: 10296 L: 9974 D: 18706
Ptnml(0-2): 135, 4504, 9902, 4798, 149
Passed LTC: https://tests.stockfishchess.org/tests/view/678d5891d63764e34db49731
LLR: 2.95 (-2.94,2.94) <0.50,2.50>
Total: 275772 W: 70655 L: 69836 D: 135281
Ptnml(0-2): 217, 30615, 75394, 31452, 208
closes https://github.com/official-stockfish/Stockfish/pull/5813
Bench: 2475787