fix: marshal attribute-only Pub/Sub messages

[winklemad]

Summary

• recognize valid Pub/Sub push messages that contain attributes without data
• preserve the absence of data while marshalling legacy-event and CloudEvent requests
• add request-path regression coverage for both signature types

Pub/Sub messages may contain either non-empty data or at least one attribute. The raw-request guard previously required truthy message.data, causing valid attribute-only messages to bypass Pub/Sub marshalling.

Testing

• npm test -- --grep 'attribute-only Pub/Sub'
• npm test -- --grep 'Pub.?Sub|pubsub'
• npm --cache /tmp/oss-public-batch/npm-cache-c3 test
• npm run compile
• npm run check
• npm run docs
• Functions Framework conformance client v1.8.3 for legacy-event and CloudEvent signatures