fix(rpc): handle rpc request with empty params

[tomonari-t]

I re-created from #122

Why this PR is needed

By JSON-RPC 2.0 Specification, params member may be omitted.

But I send below request, application is crushed.

% curl -X POST -H "Content-Type: application/json" --data '{"jsonrpc":"2.0","id":1,"method":"web3_clientVersion", "id": 1}' http://localhost:8545

[coveralls]

Coverage increased (+0.004%) to 92.708% when pulling aab5b24 on handle-rpc-params-empty into cc81d4e on master.

[coveralls]

Coverage increased (+0.004%) to 92.708% when pulling aab5b24 on handle-rpc-params-empty into cc81d4e on master.

[holgerd77]

Great, looks good, thanks for the contribution! 😄

[tomonari-t]

Thanks for reviewing!