BUG: fix empty groupby(..., as_index=False).apply(...) index name inconsistency

[Ashish-ml-eng]

Closes #48135

What happened

DataFrame.groupby(..., as_index=False).apply(...) behaved differently for empty vs non-empty input:

• non-empty input -> result.index.names == [None]
• empty input -> result.index.names == ['A'] (group key name)

What this PR changes

In the empty-values path of DataFrameGroupBy._wrap_applied_output
(pandas/core/groupby/generic.py), this PR makes as_index=False use
res_index = None instead of self._grouper.result_index.

This keeps empty-output behavior consistent with non-empty output when
as_index=False.

Tests

Added a regression test in pandas/tests/groupby/test_apply.py that checks:

• non-empty case index names are [None]
• empty case index names match the non-empty case

[Copilot]

Pull request overview

Fixes an inconsistency in DataFrame.groupby(..., as_index=False).apply(...) where empty inputs previously produced an index with the group key name (e.g. ['A']), while non-empty inputs produced unnamed index levels ([None]).

Changes:

• Adjust DataFrameGroupBy._wrap_applied_output empty-values handling to set res_index=None when as_index=False.
• Add a regression test asserting index name parity between empty and non-empty groupby(..., as_index=False).apply(...).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
pandas/core/groupby/generic.py	Aligns empty-apply index construction with the non-empty as_index=False behavior by avoiding result_index in the empty path.
pandas/tests/groupby/test_apply.py	Adds coverage to ensure empty and non-empty results have consistent index.names when as_index=False.

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