[Ingest Manager] Fixed nil pointer during unenroll

[michalpristas]

What does this PR do?

Fixes issue with unenroll serialization when nil pointer prevented unenrollment to be finished and caused agent to get into restart loop

Why is it important?

Makes agent unusable after unenroll. Unenroll is not acked so agent appears online

Fixes: #23593

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• 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.

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[elasticmachine]

Pinging @elastic/agent (Team:Agent)

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: Pull request #23609 updated

• • Start Time: 2021-01-21T12:32:46.844+0000

• Duration: 23 min 16 sec

• Commit: 22ffea7

Test stats 🧪

Test	Results
Failed	0
Passed	1458
Skipped	4
Total	1462

💚 Flaky test report

Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	1458
Skipped	4
Total	1462

[michalpristas]

/package

[ph]

@michalpristas Is there an issue that should be linked to this PR?

[blakerouse]

Looks good.