[C++] Dictionary is unset when calling DictionaryArray.GetScalar for null values

[asfimport]

Which fails the MakeArrayFromScalar(arr.GetScalar()) roundtrip.

Reporter: Krisztian Szucs / @kszucs
Assignee: Krisztian Szucs / @kszucs

Related issues:

• [C++][Python] Refactor python to arrow conversions based on a reusable conversion API (is fixed by)

_{Note: This issue was originally created as ARROW-9996. Please see the migration documentation for further details.}