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GH-40913: [C++] Fix compile warning with 'implicitly-defined constructor does not initialize' in encoding_benchmark (#41060)



### Rationale for this change
Fix the compile warning with `gcc-9.3` in encoding_benchmark.cc

### What changes are included in this PR?
Add value initialization which would have no cost for constexpr function's evaluating during non-hotpath of benchmark, and this fixed the issue for constexpr initialization in gcc 9.3.0

### Are these changes tested?
Needn't

### Are there any user-facing changes?
No

* GitHub Issue: #40913

Authored-by: hugo.zhang <hugo.zhang@openpie.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>