[Bug]: Incongruence in JSON-RPC Compliance Test

### What happened?

This compliance [test](https://github.com/a2aproject/a2a-tck/blob/9893cfdb5f5ab661958c3edcfad43be806ad5626/tests/mandatory/jsonrpc/test_json_rpc_compliance.py#L56) presents two incongruences in my opinion:

1. The test case checks for the following payload "params": {"":"not_a_dict"} -> I think it should be changed to "params": {"not_a_dict"}
2. The A2A [spec](https://a2a-protocol.org/latest/specification/#6111-jsonrpcrequest-object) does not mention that a params object containing an empty key ("") is considered an invalid format.

My point is that the test seems to be checking for an empty key as an error, but this is not explicitly forbidden by the spec. A better test for an invalid params format would be to provide a value that is not a Structured Value.

### Relevant log output

```shell

```

### Code of Conduct

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