Fix flaky test CacheStatsAPIIndicesRequestCacheIT.testNullLevels() (o…

…pensearch-project#13457) (opensearch-project#13475)

* Fix flaky test

* Initialize CommonStatsFlags with empty array for levels

* Fixes tests using incorrect null levels

* rerun

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(cherry picked from commit 5d61ac2)

Signed-off-by: Peter Alfonsi <[email protected]>
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Peter Alfonsi <[email protected]>
(cherry picked from commit 291f686)