[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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