[C++] Change BaseBinaryBuilder::GetView to return a view with a null data pointer for entries added via AppendNull

[andishgar]

Describe the enhancement requested

arrow::BaseBinaryBuilder::GetView returns an empty string for data that is added via arrow::BaseBinaryBuilder::AppendNull. This behavior has already been identified as problematic in issue 39635, where null data is incorrectly treated as an empty string, as seen here.

Component(s)

C++

[andishgar]

@pitrou, is it okay to change GetView's behavior for null data? I'm planning to apply the patch below – already ran all tests, all pass.

1b3c02.zip

[pitrou]

@andishgar Just like with BaseBinaryArray and other concrete array types, the implicit assumption is that GetValue (or GetView) should only be called for non-null slots. Perhaps this should be clarified in the respective docstrings?

[andishgar]

@pitrou, the problem is that BaseBinaryBuilder does not have any method in its public API to check whether data is null. Should we add such a method?