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

[HyukjinKwon]

Describe the enhancement requested

https://github.com/apache/arrow/blob/9cde706419c8e9cde0647af1fd55f6a86ecdbd2a/cpp/src/arrow/compute/kernels/vector_array_sort.cc#L240C5-L240C41

Component(s)

C++

[pitrou]

Issue resolved by pull request 48955
#48955