GH-44581: [C++] Minor: ArrayData ctor can assign null_count directly

[mapleFU]

Rationale for this change

See https://godbolt.org/z/37reoKTfK

The ArrayData ctor can assign null_count directly, rather than SetNullCount.

What changes are included in this PR?

Change null_count assigned directly

Are these changes tested?

Covered by existing

Are there any user-facing changes?

No

• GitHub Issue: [C++] Minor: ArrayData ctor can assign null_count directly than copying a more around #44581

[github-actions]

⚠️ GitHub issue #44581 has been automatically assigned in GitHub to PR creator.

[kou]

+1