[panw] Make event.original optional

[marc-gr]

What does this PR do?

Makes event.original optional

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
  - [ ] If I'm introducing a new feature, I have modified the Kibana version constraint in my package's manifest.yml file to point to the latest Elastic stack release (e.g. ^7.13.0).

Related issues

• Relates event.original optionality across all packages #777
• Relates Move Security integrations to GA #961

Screenshots

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Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[elasticmachine]

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Build stats

• Build Cause: Branch indexing

• Start Time: 2021-05-26T22:28:37.042+0000

• Duration: 15 min 9 sec

• Commit: d39c971

Test stats 🧪

Test	Results
Failed	0
Passed	27
Skipped	0
Total	27

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[P1llus]

LGTM, would be nice with a ECS version bump as well.