Add bucket_nullable argument for Eventstats

[ishaoxy]

Description

Similar with Stats, now we add bucket_nullable argument for Eventstats, to control whether it consider null buckets as a valid group in group-by aggregations.

Related Issues

Resolves #4801

Check List

• New functionality includes testing.
• New functionality has been documented.
• New functionality has javadoc added.
• New functionality has a user manual doc added.
• New PPL command checklist all confirmed.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff or -s.
• Public documentation issue/PR created.

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.

[LantaoJin]

@ishaoxy please resolve the conflicts

[ishaoxy]

Conflicts are resolved. @LantaoJin

[opensearch-trigger-bot]

The backport to 2.19-dev failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/sql/backport-2.19-dev 2.19-dev
# Navigate to the new working tree
pushd ../.worktrees/sql/backport-2.19-dev
# Create a new branch
git switch --create backport/backport-4817-to-2.19-dev
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 1ecf42d5028bc3bdefffff5ecec8cbd81a471aa4
# Push it to GitHub
git push --set-upstream origin backport/backport-4817-to-2.19-dev
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/sql/backport-2.19-dev

Then, create a pull request where the base branch is 2.19-dev and the compare/head branch is backport/backport-4817-to-2.19-dev.

[LantaoJin]

@ishaoxy need to backport manually