[Heartbeat] Add nil check to diagnostics processor serializer

[emilioalvap]

What does this PR do?

Closes #35635.
Related to #35031.

TL;DR: We are seeing segfaults when running elastic-agent diagnostics on heartbeat policies. Traced back to a nested nil access. AFAIK, there're other beats affected by the same problem.

• Add nil check to diagnostics processor serializer.

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.

How to test this PR locally

• Build agent locally.
• Enroll an agent in a synthetics policy.
• Run elastic-agent diagnostics.
• Check no segfault error is reported and .zip contains parsed synthetic policies.

[elasticmachine]

Pinging @elastic/uptime (Team:Uptime)

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[elasticmachine]

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Build stats

• Start Time: 2023-06-07T16:04:25.489+0000

• Duration: 71 min 17 sec

Test stats 🧪

Test	Results
Failed	0
Passed	27307
Skipped	2007
Total	29314

💚 Flaky test report

Tests succeeded.

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