Support prunning on the newly supported NullCount

[AdamGS]

Currently panicks in some benchmarks, follows #4980.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.27%. Comparing base (efa9f4d) to head (d3a6222).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
vortex-file/src/pruning.rs	50.00%	1 Missing ⚠️

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