GH-43838: [Python][C++] Preserve nested NullType metadata in array views

[dartkron]

Rationale for this change

ViewDataImpl skipped input layout entries marked ALWAYS_NULL. A nested
NullType has no physical buffer, but its ArrayData still carries the length
and offset needed for the output. Losing this metadata could make a cast to the
same nested type produce an invalid child array.

Fixes #43838.

What changes are included in this PR?

Preserve length and offset when matching an input NullType with an output
NullType, while retaining the previous behavior for other ALWAYS_NULL
layouts, including union and run-end encoded types. Add a regression test for
a sliced nested NullArray.

Are these changes tested?

Yes. The regression test and all 51 CTest targets enabled in the local C++
build pass in an Ubuntu 24.04 container.

Are there any user-facing changes?

Yes. Casts containing nested NullType fields no longer produce invalid child
arrays. There are no public API changes.

AI assistance

AI assistance was used during investigation, implementation, and review. I
reviewed the final changes and verified them with the tests described above.

This PR contains a "Critical Fix". The previous behavior could produce an
invalid array from valid input.

• GitHub Issue: [Python][C++] Casting a list of struct array with null field results in invalid result #43838

[github-actions]

⚠️ GitHub issue #43838 has been automatically assigned in GitHub to PR creator.

[github-actions]

⚠️ GitHub issue #43838 has been automatically assigned in GitHub to PR creator.