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ARROW-14050: [C++] Make TDigest/Quantile kernels return nulls instead

As requested, this makes it so that TDigest/Quantile return nulls instead of empty arrays, making them easier to consume from bindings, and making them compose better with kernels like list_flatten and list_element.

Closes #11199 from lidavidm/arrow-14050

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>