Add ignore_empty_values to the append processor

[joegallo]

In 9.2, the append processor learned a new trick -- it can ignore_empty_values.

I'll probably learn how backports work while merging #5861. I noticed that issue while I was coming here to fix this issue. I did them as separate PRs because they're going to have different backporting destinations.

elastic/elasticsearch#105718

[github-actions]

Following you can find the validation changes against the target branch for the APIs.

No changes detected.

You can validate these APIs yourself by using the make validate target.

[pquentin]

Thanks! LGTM.

(This time there is a YAML test, but we don't currently use the ingest-common module.)

[github-actions]

The backport to 9.2 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-9.2 9.2
# Navigate to the new working tree
cd .worktrees/backport-9.2
# Create a new branch
git switch --create backport-5862-to-9.2
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 343f84c2be472ac9228a824fb3bc7a05263dd498
# Push it to GitHub
git push --set-upstream origin backport-5862-to-9.2
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-9.2

Then, create a pull request where the base branch is 9.2 and the compare/head branch is backport-5862-to-9.2.

[pquentin]

💚 All backports created successfully

Status	Branch	Result
✅	9.2

Questions ?

Please refer to the Backport tool documentation