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ARROW-16043: [C++][Filesystem][S3] Add missing empty content for creating directory

We can't omit body to create a directory.

Closes #12895 from kou/cpp-s3-create-directory

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>