Improve null move pruning

[FitzOReilly]

• Only do null move pruning if the static evaluation is greater than or equal to beta
• Minor refactoring

[codecov[bot]]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (463bc58) 92.65% compared to head (76a1f73) 92.65%.

Files	Patch %	Lines
search/src/search.rs	75.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #190 +/- ## ======================================= Coverage 92.65% 92.65% ======================================= Files 83 83 Lines 13647 13664 +17 ======================================= + Hits 12644 12661 +17 Misses 1003 1003 ```

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