[system] Add missing null check in security pipeline

[marc-gr]

What does this PR do?

Fix a missing nullcheck

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).

[elasticmachine]

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

[elasticmachine]

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Build stats

• Start Time: 2021-11-10T09:46:53.154+0000

• Duration: 13 min 2 sec

• Commit: 2e5c9cd

Test stats 🧪

Test	Results
Failed	0
Passed	267
Skipped	0
Total	267

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💚 Build Succeeded

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Build stats

• Start Time: 2021-11-10T09:45:53.148+0000

• Duration: 15 min 22 sec

• Commit: b0d782a

Test stats 🧪

Test	Results
Failed	0
Passed	267
Skipped	0
Total	267

🤖 GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.