fix: add support for null/invalid type for multi-value-headers

[mbergkvist]

Issue #, if available: 367

Description of changes:

This adds support for invalid/null value for multi-value headers, defaulting to an empty HeaderMap if need be, increasing resilience if the Lambda function would get an invalid value for multi-value headers.

This is a continuation of the work done by @nmoutschen in #371.

I have also successfully run the test in #367 (comment) locally.

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• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I confirm that I've made a best effort attempt to update all relevant documentation.

[bahildebrand]

LGTM