GH-49889: [C++][Compute] Handle logical nulls in validity kernels

[hadrian-reppas]

Rationale for this change

I was looking into #49889 and updated the handling of logical nulls in the is_valid, is_null, and true_unless_null kernels. It turns out there is some prior work here that I didn't see before I starting implementing my fix (#35058, #35036, #37642). For some reason, that work has stalled.

What changes are included in this PR?

This PR updates the IsValidExec, IsNullExec, and TrueUnlessNullExec kernels so that they call out to a new function SetLogicalNullBits. The SetLogicalNullBits function matches the logic in ArraySpan::ComputeLogicalNullCount(). The logic is similar enough that they may be some opportunities for code deduplication in dict_util.cc and ree_util.cc.

Are these changes tested?

Yes

Are there any user-facing changes?

No

• GitHub Issue: [C++][Compute] true_unless_null kernel output incorrect when run-end encoded array contains null #49889

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