[C++] Fix handling of logical nulls by compute kernels processing run-end encoded arrays or arrays of unions

[felipecrv]

Since #34361 and the introduction of special validity checks in #34408, we are aware that parts of the codebase handle nulls the wrong way when the input is a run-end encoded array (NEW) or an union array.

This issue aims to be a list of compute kernels that have to be changed regarding nulls in these special array types by using specialized ways of determining a valid is a logical null instead of relying on validity bitmaps that are absent in these types of arrays:

• is_null [Python] pa.compute.is_null() returns incorrect answer for dense union arrays and segfaults for dense union scalars #34315
• hash_count [C++] hash_count kernel miscounts when run-end encoded array contains null #35059
• ...

Component(s)

C++

[felipecrv]

cc @jorisvandenbossche @westonpace