ARROW-11970: [C++][CI] Fix Valgrind error in arrow-csv-test
ARROW-11970: [C++][CI] Fix Valgrind error in arrow-csv-test

Fix the uninitialized read error when trying to represent the write test parameters.

The uninitialized read error due to fast_float will be separately fixed by bumping the vendored fast_float version (ARROW-11833).

Closes #9711 from pitrou/ARROW-11833-csv-valgring

Authored-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>
Signed-off-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>

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