Fix error when APIGateway headers are None

[sfirrin]

What does this PR do?

Fixes #249 by allowing context["headers"] to be None and falling back to an empty {}

Motivation

#249

Testing Guidelines

Passes tests and added another for this case

Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

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• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
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