Allow Pv nodes at certain conditions to spawn zero window searches deeper than default
In current case it's allowed if there is no best move.
Passed STC:
https://tests.stockfishchess.org/tests/view/67640fd586d5ee47d9543d5a
LLR: 2.96 (-2.94,2.94) <0.00,2.00>
Total: 392480 W: 102038 L: 101192 D: 189250
Ptnml(0-2): 1303, 46287, 100253, 47055, 1342
Passed LTC:
https://tests.stockfishchess.org/tests/view/67671a4686d5ee47d9544476
LLR: 2.94 (-2.94,2.94) <0.50,2.50>
Total: 128616 W: 32941 L: 32433 D: 63242
Ptnml(0-2): 84, 13997, 35634, 14513, 80
closes https://github.com/official-stockfish/Stockfish/pull/5733
Bench: 1095871