Fix falsy Grouper key and level handling

[fallintoplace]

What

• Check Grouper key and level selectors against None.
• Keep empty-string keys and level 0 on their intended paths.

Why

• Truthiness checks skip valid key="" and level=0 values.

Tests

• Add resampling coverage for on="" and level=0.
• Add groupby coverage for Grouper(key="").
• Applicable pre-commit checks pass.

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📥 Commits

Reviewing files that changed from the base of the PR and between 9a601e2 and b3fb5ee.

📒 Files selected for processing (4)

• python/cudf/cudf/core/groupby/groupby.py
• python/cudf/cudf/core/resample.py
• python/cudf/cudf/tests/groupby/test_reductions.py
• python/cudf/cudf/tests/groupby/test_resample.py

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📝 Summary

Summary by CodeRabbit

• Bug Fixes

  • Fixed grouping with empty-string column names.
  • Fixed resampling when using empty labels or the first level of a MultiIndex.
  • Improved consistency with pandas for grouping and daily resampling results.

• Tests

  • Added coverage for empty column names, empty grouping keys, and MultiIndex-based resampling.

Walkthrough

Changes

The grouper and resample paths now treat non-None keys and levels as explicitly provided. New tests cover empty column names and MultiIndex level-based resampling.

Grouper routing

Layer / File(s)	Summary
Preserve explicit grouper values python/cudf/cudf/core/groupby/groupby.py, python/cudf/cudf/core/resample.py	Groupby and resample routing now uses None checks instead of truthiness checks for keys and levels.
Validate falsey and level-based grouping python/cudf/cudf/tests/groupby/test_reductions.py, python/cudf/cudf/tests/groupby/test_resample.py	Tests compare pandas and cuDF results for an empty-string column name and MultiIndex level-zero resampling.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix

Merge Risk: ⚪ Minimal · up to b3fb5

The change fixes empty-string and level-zero Grouper handling without an established regression, and the affected paths have pandas-parity coverage.

🚥 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%. Docstring coverage is scoped to functions touched by this diff. Analyzed 9 functions across 4 files.	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 clearly identifies the main change: fixing handling for falsy Grouper keys and levels.
Description check	✅ Passed	The description accurately explains the None checks, affected falsy values, rationale, and added tests.
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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[mroeschke]

/ok to test b3fb5ee