GH-48954: [C++] Add test for null-type dictionary sorting and clarify XXX comment

[HyukjinKwon]

Rationale for this change

Null-type dictionaries (e.g., dictionary(int8(), null())) are valid Arrow constructs supported from day one, but the sorting code had an uncertain XXX Should this support Type::NA? comment. We should explicitly support and test this because other functions already support this:

import pyarrow as pa
import pyarrow.compute as pc

pc.array_sort_indices(pa.array([None, None, None, None], type=pa.int32()))
# [0, 1, 2, 3]
pc.array_sort_indices(pa.DictionaryArray.from_arrays(
    indices=pa.array([None, None, None, None], type=pa.int8()),
    dictionary=pa.array([], type=pa.null())
))
# [0, 1, 2, 3]

I believe it does not make sense to specifically disallow this in dictionaries at this point.

What changes are included in this PR?

Added a unittest for null sorting behaviour.

Are these changes tested?

Yes, the unittest was added.

Are there any user-facing changes?

No.

• GitHub Issue: [C++] Add test for null-type dictionary sorting and clarify XXX comment #48954

[github-actions]

⚠️ GitHub issue #48954 has been automatically assigned in GitHub to PR creator.

[raulcd]

I understand we add a test to validate it doesn't blow up or produce garbage even though there's no usefulness to it, right?
I was just trying to understand what does that even mean (the null-type dictionary sorting case).