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GH-45304: [C++][S3] Workaround compatibility issue between AWS SDK and MinIO (#45310)

### Rationale for this change

Some AWS SDK versions have faulty chunked encoding when the body is 0 bytes:
https://github.com/aws/aws-sdk-cpp/issues/3259

### What changes are included in this PR?

Work around faulty chunked encoding implementation by only setting a body stream if non-empty.

### Are these changes tested?

Locally for now, but will be picked by CI (and conda-forge) at some point.

### Are there any user-facing changes?

No.

* GitHub Issue: #45304

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