[C++] arrow::ScalarEquals returns false when values are both null

[asfimport]

I wasn't sure if this was deliberate but it appeared while writing unit tests and so wanted to check what was the intention before changing it. Arrays compare equal when null slots are respectively null so this seems inconsistent.

Reporter: Wes McKinney / @wesm
Assignee: Antoine Pitrou / @pitrou

Related issues:

• [C++] Support comparing scalars approximately (is superceded by)

_{Note: This issue was originally created as ARROW-8956. Please see the migration documentation for further details.}

[asfimport]

Neal Richardson / @nealrichardson:
Wouldn't it make more sense (for some mental model of what missing data is) for them both to return null instead?

$ R
> NA == NA
[1] NA
> NaN == NaN
[1] NA

[asfimport]

Wes McKinney / @wesm:
The only options for this function at the moment are true or false

https://github.com/apache/arrow/blob/master/cpp/src/arrow/compare.h#L114

This is data structure equality, not elementwise analytic equality