arrow

GH-41955: [C++] Follow up of adding null_bitmap to MapArray::FromArrays (#41956)

### Rationale for this change

There have been some new comments regarding the work done in https://github.com/apache/arrow/pull/41757.

### What changes are included in this PR?

This PR addresses the comments from https://github.com/apache/arrow/pull/41757#pullrequestreview-2094287563

### Are these changes tested?

Yes. Existing tests should pass.

### Are there any user-facing changes?

No.
* GitHub Issue: #41955

Lead-authored-by: AlenkaF <frim.alenka@gmail.com>
Co-authored-by: Alenka Frim <AlenkaF@users.noreply.github.com>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Signed-off-by: Felipe Oliveira Carvalho <felipekde@gmail.com>