[C++] `IsNull` produces incorrect results on sliced sparse union arrays

[hadrian-reppas]

Describe the bug, including details regarding any error messages, version, and platform.

  ArrayVector children{ArrayFromJSON(int64(), "[1, 2]"),
                       ArrayFromJSON(int8(), "[null, 3]")};
  auto type_ids = ArrayFromJSON(int8(), "[0, 1]");
  auto type = sparse_union({field("a", int64()), field("b", int8())});
  auto array =
      std::make_shared<SparseUnionArray>(type, 2, children, type_ids->data()->buffers[1]);
  auto slice = array->Slice(1);

  ASSERT_FALSE(array->IsNull(0));
  ASSERT_FALSE(array->IsNull(1));
  ASSERT_FALSE(slice->IsNull(0));  // This should succeeded, but it fails

  ASSERT_OK_AND_ASSIGN(auto expected, array->GetScalar(1));
  ASSERT_OK_AND_ASSIGN(auto actual, slice->GetScalar(0));
  AssertScalarsEqual(*expected, *actual);  // This should succeeded, but it fails

Component(s)

C++

[hadrian-reppas]

I'm working on a fix now

[hadrian-reppas]

Never mind this was fixed by #50108