Skip null buffer when importing FFI ArrowArray struct if no null buffer in the spec

[viirya]

Describe the bug

See #3276 (comment).

Currently we load the buffer at index 0 of buffers fields from FFI ArrowArray struct. However, it could load wrong buffer as null buffer for the types where is no null buffer, e.g., UnionArray.

To Reproduce

Expected behavior

Additional context

C++ Arrow exporter: https://github.com/apache/arrow/blob/26a426f325256e260a15521d5097efffd2f1ceb1/cpp/src/arrow/c/bridge.cc#L556C14-L566

[viirya]

Actually we may need to exclude null buffer exporting for such types too.