GH-49928: [C++][Parquet] Fix UB in UpdateLevelHistogram from nullptr std::span

[Diveyam-Mishra]

[Parquet] Guard against null level pointers in UpdateLevelHistogram

column_writer.cc

1. Added ARROW_DCHECK and null guard in the add_levels lambda
   size_statistics_test.cc
2. Added regression test for empty/null span

Add an explicit null-pointer check before constructing a std::span in
TypedColumnWriterImpl::UpdateLevelHistogram. This avoids relying on the
implicit invariant that nullptr level pointers only occur when
max_level == 0.

Also add a regression test verifying that UpdateLevelHistogram handles
an empty span as a no op.

• GitHub Issue: [Parquet] UB in TypedColumnWriterImpl::UpdateLevelHistogram due to std::span construction from nullptr #49928

[github-actions]

⚠️ GitHub issue #49928 has been automatically assigned in GitHub to PR creator.

[Copilot]

Pull request overview

Fixes undefined behavior in Parquet C++ column writing by preventing construction of a std::span from a potentially null level pointer during size-statistics histogram updates.

Changes:

• Add a null-pointer guard (and debug assertion) in TypedColumnWriterImpl::UpdateLevelHistogram before constructing a std::span.
• Add a regression test ensuring parquet::UpdateLevelHistogram treats an empty std::span as a no-op.

Reviewed changes

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

File	Description
cpp/src/parquet/column_writer.cc	Adds null-guard/DCHECK around level histogram updates to avoid std::span UB.
cpp/src/parquet/size_statistics_test.cc	Adds regression coverage for empty-span histogram updates.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.