[C++] Address missed code review comments in ordered aggregation

[icexelloss]

Describe the enhancement requested

A couple of review comments are missed about during #34311

Created this issue to track this minor follow up

Component(s)

C++

[icexelloss]

e.g. https://github.com/apache/arrow/pull/34311/files/d68efd4c295d9fedfaf6ccc7804dfb591c4ecff2#r1128845883

[icexelloss]

nvm - github diff issue