[Bug]: Clarify how HTTP+JSON/REST returns google.protobuf.Empty

### What happened?

Currently, the A2A specification for the HTTP+JSON/REST binding does not explicitly specify how to return successful empty responses (for operations that return google.protobuf.Empty in spec proto).

Binding requirement states that `Content-Type: application/json` is used for both requests and responses, does this mean with we should return an empty object {}? Or should it maybe be 204 No Content result.

### Relevant log output

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### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Look at the A2A specification for the HTTP+JSON/REST binding, likely in a spec or documentation directory. Find where google.protobuf.Empty return types are handled. Examine existing implementations or tests for similar operations to see current behavior. Determine whether an empty JSON object {} or a 204 No Content status is appropriate, and propose a clarification in the spec.

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