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ARROW-16048: [Python] Avoid exposing null buffer address to the Python buffer protocol

A 0-size buffer created in C++ can very well have a null address. However, when exporting that buffer through the Python buffer API, we should ensure we pass a valid pointer.

Also, ensure mutability of a buffer is preserved when pickling and unpickling.

Closes #12752 from emkornfield/fix_bug

Lead-authored-by: Antoine Pitrou <antoine@python.org>
Co-authored-by: Micah Kornfield <micahk@google.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>