ARROW-12420: [C++/Dataset] Reading null columns as dictionary not longer possible

[kszucs]

No description provided.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-12420

[kszucs]

I reproduced the problem using a simple cast test from null array to a dictionary array:

TEST(Cast, FromNullToDictionary) {
  auto from = std::make_shared<NullArray>(10);
  auto to_type = dictionary(int8(), boolean());

  ASSERT_OK_AND_ASSIGN(auto expected, MakeArrayOfNull(to_type, 10));
  CheckCast(from, expected);
}

Somewhere ArrayData::dictionary gets set to nullptr or not set at all. I don't have a solution yet, it's a but hard to debug due to the Datum variant.

cc @bkietz

[wesm]

+1 here, merge on green build