Improve nil handling, add tests to serverless implementation

[fearful-symmetry]

Proposed commit message

This fixes an issue where a nil config can cause a panic, and adds a variety of tests to make sure we don't run into this again.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• 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]

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