arrow

GH-44581: [C++] Minor: ArrayData ctor can assign null_count directly (#44582)



### 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: #44581

Authored-by: mwish <maplewish117@gmail.com>
Signed-off-by: mwish <maplewish117@gmail.com>