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ARROW-10928: [C++] Better Parquet error when trying to write empty struct

An empty struct type (with no child fields) is not easy to write in Parquet,
since Parquet only represents the data of leaf nodes.
We would need a way to distinguish between null and non-null (empty) struct values.
It would probably require a dummy primitive node.

Until we implement such a solution, simply raise a nice error when an empty
struct is encountered.

Closes #8939 from pitrou/ARROW-10928-parquet-empty-struct

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