[8.x](backport #41198) Create nil cgroups reader for processor, improve error handling

[mergify]

Proposed commit message

Continuation of #41189 . This adds a NilCGReader type that makes us a little less likely to hit a nil pointer deref if this code is changed in the future, and also cleans up some of the error handling.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

---

This is an automatic backport of pull request #41198 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 764ba96 has failed:

On branch mergify/bp/8.x/pr-41198
Your branch is up to date with 'origin/8.x'.

You are currently cherry-picking commit 764ba96d34.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   filebeat/autodiscover/defaults_aix.go
	both modified:   libbeat/processors/add_docker_metadata/add_docker_metadata.go
	both modified:   libbeat/processors/add_process_metadata/add_process_metadata.go
	both modified:   libbeat/processors/add_process_metadata/add_process_metadata_test.go
	both modified:   libbeat/processors/add_process_metadata/gosigar_cid_provider.go

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[botelastic]

This pull request doesn't have a Team:<team> label.

[pierrehilbert]

@fearful-symmetry could you please fix the conflicts here?

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b mergify/bp/8.x/pr-41198 upstream/mergify/bp/8.x/pr-41198
git merge upstream/8.x
git push upstream mergify/bp/8.x/pr-41198

[mergify]

This pull request has not been merged yet. Could you please review and merge it @fearful-symmetry? 🙏