fix: is_sorted returns nullable bool

[danking]

No description provided.

[codspeed-hq]

CodSpeed Performance Report

Merging #4555 will degrade performances by 15.4%

_{Comparing dk/fix-is-sorted-nullable-bool (300ac82) with develop (425b55a)}

Summary

❌ 1 regressions
✅ 1350 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
❌	encode_specialized	1.1 ms	1.3 ms	-15.4%

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.62%. Comparing base (425b55a) to head (300ac82).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
vortex-array/src/compute/is_sorted.rs	0.00%	2 Missing ⚠️

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