fix(openapi): correct response validation for falsy objects

[amin-farjadi]

Issue number: closes #5887, #6216

Summary

Fix openapi response validation behaviour on Falsy return objects.

Changes

in openapi_validation.py, OpenAPIResponseValidationMiddleware class:

• behaviour of _handle_response method modified
• behaviour of _serialize_response method modified
• _serialize_response method renamed to _serialize_response_with_validation

in test_openapi_validation_middleware.py:

• remove the skip from test_validation_error_none_returned_non_optional_type to cover issue Bug: Open API Validation not validating response serialization when body is Falsy. #5887
• added test_validation_error_different_list_returned_non_optional_type and test_validate_list_response to cover Bug: Upgrade to 3.7.0 breaks List response validation in APIGatewayRestResolver #6216

in test_http_resolver_pydantic.py:

• test_async_handler_with_validation marked to skip due to issue Bug(HttpResolverLocal): OpenAPIResponseValidationMiddleware is not working as intended #7981

User experience

Now Falsy response will be validated againt the OpenAPI return field (is there is any).

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.63%. Comparing base (c4434b7) to head (6466bbf).

Files with missing lines	Patch %	Lines
...ls/event_handler/middlewares/openapi_validation.py	88.88%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7990      +/-   ##
===========================================
- Coverage    96.73%   96.63%   -0.10%     
===========================================
  Files          278      278              
  Lines        13654    13656       +2     
  Branches      1086     1086              
===========================================
- Hits         13208    13197      -11     
- Misses         327      338      +11     
- Partials       119      121       +2     

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