Only apply null move pruning in expected cut nodes

[TheRealGioviok]

Passed STC:

Test  | cutnodenmp
Elo   | 1.81 +- 1.45 (95%)
SPRT  | 8.0+0.08s Threads=1 Hash=16MB
LLR   | 2.98 (-2.94, 2.94) [0.00, 3.00]
Games | N: 73418 W: 18812 L: 18429 D: 36177
Penta | [807, 8838, 17067, 9159, 838]

https://ob.cwchess.org/test/1347/

Passed LTC:

Test  | cutnodenmp
Elo   | 2.90 +- 2.07 (95%)
SPRT  | 40.0+0.40s Threads=1 Hash=64MB
LLR   | 2.95 (-2.94, 2.94) [0.00, 3.00]
Games | N: 29384 W: 7239 L: 6994 D: 15151
Penta | [114, 3416, 7401, 3633, 128]

https://ob.cwchess.org/test/1348/

Bench: 18290913