double nil return in grpc query handler crash the node when using GRPCClient

[yihuang]

Summary of Bug

We updated ethermint to use GRPCClient(introduced here) to enjoy concurrency, but there's a query handler that return nil, nil under certain conditions, when that happens, it'll crash the whole node.
Currently, we fix that by returning a proper error, but since crashing the node could be a DoS vulnability, it'd be good that we eliminate that possibility.

the panic happens here, I guess the nil value is not caught by the previous check as the comment suggested.

Version

0.46

Steps to Reproduce