okta: handle already set event.original more robustly

[efd6]

What does this PR do?

This ensures that already-set event.original documents can still be ingested.

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.
• I have verified that Kibana version constraints are current according to guidelines.

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Closes [Okta] Integration can throw the error field [event.original] already exists #4782

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• Start Time: 2022-12-14T09:07:09.997+0000

• Duration: 14 min 56 sec

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Test	Results
Failed	0
Passed	7
Skipped	0
Total	7

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (1/1)	💚
Files	100.0% (1/1)	💚
Classes	100.0% (1/1)	💚
Methods	100.0% (17/17)	💚 6.667
Lines	86.111% (496/576)	👎 -13.889
Conditionals	100.0% (0/0)	💚

[elasticmachine]

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

[kcreddy]

Minor clarification. LGTM 👍🏼

[elasticmachine]

Package okta - 1.12.0 containing this change is available at https://epr.elastic.co/search?package=okta