fix(lambda-events): omit null IoT policy documents - #1168

📬 Issue #, if available:

Fixes #1131.

✍️ Description of changes:

AWS IoT rejects custom authorizer responses containing null entries in
policyDocuments.

This change keeps the existing Vec<Option<IamPolicyDocument>> public type to
preserve backwards compatibility and adds a custom serializer that omits
None entries while preserving the order of the remaining policy documents.

Regression tests cover:

• a single policy document
• mixed Some and None entries
• a vector containing only None
• an empty vector
• unchanged deserialization of null policy documents

Validation completed:

• cargo +nightly fmt --all -- --check
• cargo test --package aws_lambda_events --no-default-features --features iot
• cargo +stable clippy --workspace --all-features -- -D warnings

🔏 By submitting this pull request

• I confirm that I've ran cargo +nightly fmt.
• I confirm that I've ran cargo clippy --fix.
• I confirm that I've made a best effort attempt to update all relevant documentation.
• I confirm that my contribution is made under the terms of the Apache 2.0 license.