[zeek] Make event.original optional

[marc-gr]

What does this PR do?

Make 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.

Related issues

• Relates event.original optionality across all packages #777

Screenshots

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

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Build stats

• Build Cause: Pull request #992 updated

• Start Time: 2021-06-08T08:38:53.661+0000

• Duration: 33 min 50 sec

• Commit: 357d9c5

Test stats 🧪

Test	Results
Failed	0
Passed	155
Skipped	0
Total	155

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[P1llus]

LGTM, small nitpick, you could change the description for:
description: Drop if no Splunk or log data present.
To the fact that it only drops if the data is from Splunk, since there is no empty values from zeek directly.