Fix flaky test CacheStatsAPIIndicesRequestCacheIT.testNullLevels() (opensearch-project#13457) (opensearch-project#13475)

* Fix flaky test

* Initialize CommonStatsFlags with empty array for levels

* Fixes tests using incorrect null levels

* rerun

---------

(cherry picked from commit 5d61ac2)

Signed-off-by: Peter Alfonsi <petealft@amazon.com>
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 <petealft@amazon.com>
(cherry picked from commit 291f686)

Author: 3 people