Fix mypy arg-type for from_buffers after pyarrow-stubs bump

[madsbk]

Description

With zen-xu/pyarrow-stubs#284, I started seeing CI errors like:

full log

mypy.....................................................................Failed
- hook id: mypy
- exit code: 1

python/cudf/cudf/core/column/column.py:592: error: Argument 3 to "from_buffers" of "Array" has incompatible type "list[Buffer]"; expected "list[Buffer | None]"  [arg-type]
python/cudf/cudf/core/column/column.py:592: note: "list" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
python/cudf/cudf/core/column/column.py:592: note: Consider using "Sequence" instead, which is covariant
python/cudf/cudf/core/column/column.py:615: error: Argument 3 to "from_buffers" of "Array" has incompatible type "list[Buffer]"; expected "list[Buffer | None]"  [arg-type]
python/cudf/cudf/core/column/column.py:615: note: "list" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
python/cudf/cudf/core/column/column.py:615: note: Consider using "Sequence" instead, which is covariant
python/cudf/cudf/core/groupby/groupby.py:573: note: By default the bodies of untyped functions are not checked, consider using --check-untyped-defs  [annotation-unchecked]
python/cudf/cudf/core/groupby/groupby.py:596: note: By default the bodies of untyped functions are not checked, consider using --check-untyped-defs  [annotation-unchecked]
Found 2 errors in 1 file (checked 615 source files)

[madsbk]

/ok to test

[coderabbitai]

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between fc8fddb and 3d5aad1.

📒 Files selected for processing (1)

• python/cudf/cudf/core/column/column.py

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes
  • Improved handling of nested null values when working with struct and list data, helping array reconstruction behave more reliably in edge cases.

Walkthrough

_handle_nulls now types the Arrow buffer slices used to reconstruct nested StructArray and ListArray values as possibly containing None.

Changes

Null reconstruction typing

Layer / File(s)	Summary
Struct and list buffer slices python/cudf/cudf/core/column/column.py	_handle_nulls widens the buffers slice types for struct and list reconstruction from list[pa.Buffer] to list[pa.Buffer | None].

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the mypy type-fix for from_buffers after the pyarrow-stubs bump.
Description check	✅ Passed	The description directly matches the changeset and explains the CI mypy errors being fixed.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[TomAugspurger]

/merge