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ARROW-12065: [C++][Python] Fix segfault reading JSON file

A segfault would occur when a field is inferred as null in a first block and then as list in a second block.

Also re-enable `chunked_builder_test.cc`, which wasn't compiled.

Closes #9783 from pitrou/ARROW-12065-json-segfault

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