[master](backport #28219) processors/actions/add_fields: Do not panic if event.Fields is nil map

[mergify]

This is an automatic backport of pull request #28219 done by Mergify.
Cherry-pick of bef0411 has failed:

On branch mergify/bp/master/pr-28219
Your branch is up to date with 'origin/master'.

You are currently cherry-picking commit bef0411cf.
  (all conflicts fixed: 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)

nothing to commit, working tree clean

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

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[botelastic]

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[elasticmachine]

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• Start Time: 2021-10-07T07:47:42.332+0000

• Duration: 16 min 31 sec

• Commit: c28e0c4

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[jsoriano]

Closing, empty PR.