fix(aws_lambda): handle null headers, query string, and body in API Gateway events

[sahiljagtap08]

Summary

API Gateway sends null, not a missing key, for request fields that have no value. For a REST API (payload format v1) headers, multiValueHeaders, and queryStringParameters are all null when the request has none of them. The AWS Lambda adapter assumed these were always dicts and raised AttributeError inside to_bolt_request, before the request reached the middleware chain.

Two related edge cases were fixed at the same time:

• isBase64Encoded missing from the event raised KeyError. This happens when the function is invoked by test tooling or a custom integration that does not set the flag.
• isBase64Encoded: true with a null body raised TypeError from base64.b64decode.

Changes in slack_bolt/adapter/aws_lambda/handler.py:

• Every event field lookup treats None the same as a missing key.
• The base64 decode only runs when the flag is true and a body is present.
• The headers dict is copied before the cookie header is added, so the caller's event is left untouched.
• A short comment explains why the null handling is there.

Testing

New tests in tests/adapter_tests/aws/test_aws_lambda.py:

• test_null_fields_in_api_gateway_event: null headers, multiValueHeaders, and queryStringParameters now produce a 401 (no signature) instead of a crash, and the original event is not modified.
• test_missing_is_base64_encoded_and_null_body: covers the missing flag and the null body with the flag set.
• test_base64_encoded_body: a properly signed base64-encoded body is decoded and handled with a 200.

Both crash tests fail on main and pass with this change. Ran ./scripts/format.sh, ./scripts/lint.sh, ./scripts/run_mypy.sh, and the adapter test suite.

Category

• slack_bolt.App and/or its core components
• slack_bolt.async_app.AsyncApp and/or its core components
• Adapters in slack_bolt.adapter
• Others

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run ./scripts/install_all_and_run_tests.sh after making the changes.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @sahiljagtap08 to sign the Salesforce Inc. Contributor License Agreement.