fix(event-handler): handle empty proxy response bodies

[svozza]

Summary

Returning { statusCode: 204, body: '' } caused the router to return 500 because the Web Response constructor rejects a non-null body for statuses 204, 205, and 304. Empty proxy-result strings now become null before construction, preserving the requested status and empty response body.

Changes

• Normalize empty strings alongside null and omitted proxy-response bodies.
• Add buffered regressions across REST API, HTTP API, and ALB, plus streaming and empty base64-body coverage.
• Verify status 200 and omitted-body responses continue to work.
• Validate workspace test types, staged-file linting, and the unit suite with 100% coverage.

Issue number: closes #5699

---

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

Disclaimer: We value your time and bandwidth. As such, any pull requests created on non-triaged issues might not be successful.