feat: Add nulls support for rolling_mean_by

[Voultapher]

Fixes: #25848

[Voultapher]

Run curl -LsSf https://astral.sh/uv/install.sh | sh
curl: (56) The requested URL returned error: 504
Error: Process completed with exit code 56.

CI failure seems unrelated, didn't find the run again button.

[orlp]

Fixme? Safety here is trivial as 0..0 is in-bounds for any array.

[orlp]

Feel free to rework Ag::new to just check to make it safe though, it's not called often.

[yonikremer]

I fixed the issue that causes the tests to fail, update your branch an rerun the github actions and the tests should pass

[codecov]

Codecov Report

❌ Patch coverage is 52.12766% with 90 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.50%. Comparing base (fcf909e) to head (abdb381).

Files with missing lines	Patch %	Lines
...unkedarray/rolling_window/rolling_kernels/nulls.rs	39.53%	78 Missing ⚠️
...s-time/src/chunkedarray/rolling_window/dispatch.rs	79.66%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25917      +/-   ##
==========================================
- Coverage   80.51%   80.50%   -0.02%     
==========================================
  Files        1771     1772       +1     
  Lines      243271   243422     +151     
  Branches     3037     3037              
==========================================
+ Hits       195875   195955      +80     
- Misses      46614    46685      +71     
  Partials      782      782              

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