[CCI] Type improvement and optional callback on empty method

[timursaurus]

Description

Add more type safety
Make empty method's callback arg optional
Add/fix tests for listed above

Issues Resolved

Closes #452

Check List

• New functionality includes testing.
  • All tests pass
• Linter check was successfull - yarn run lint doesn't show any errors
• Commits are signed per the DCO using --signoff
• Changelog was updated.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[dblock]

Looks good. Needs USER_GUIDE and CHANGELOG updates pls.

[timursaurus]

Looks good. Needs USER_GUIDE and CHANGELOG updates pls.

On its way!

[timursaurus]

@dblock @nhtruong Any further comments or suggestions?

[dblock]

I would add a test that an empty pool can be emptied again, otherwise LGTM!

[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-2.x 2.x
# Navigate to the new working tree
cd .worktrees/backport-2.x
# Create a new branch
git switch --create backport/backport-451-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 8374dd66d5c3a80c88a78511c2b752453bc0b47f
# Push it to GitHub
git push --set-upstream origin backport/backport-451-to-2.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-451-to-2.x.