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ARROW-14179: [C++][C] Do not export/import null bitmap for union and null types

Union and Null types don't support a validity bitmap according to the buffer layout spec:
https://arrow.apache.org/docs/format/Columnar.html#buffer-listing-for-each-layout

However, Arrow C++ internally stored an empty null bitmap buffer for these types.
We must therefore be careful not to export this buffer accross the C data interface.

On import, we support both layouts for compatibility.

Closes #11280 from pitrou/ARROW-14179-c-data-export

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>