GetValidatorInformation should return error on invalid data

[wqking]

Reproduce:

1. Create a MempoolRequest.
2. Set MempoolRequest.LastBlockHash to random hash.
3. Call a Beacon node's GetValidatorInformation with the request.

The Beacon node will return success.
Expect: Beacon should report error because the request type is wrong (it should be GetValidatorRequest, but here is MempoolRequest).
This can be reproduced in Python since Python is dynamic type.