Differentiate calls with no params and calls with empty list param

[yanivmo]

This change set proposes adding support for method calls with an empty array of positional arguments.

Rationale

When issuing a call with positional arguments, the current implementation doesn't differentiate between the calls missing the params property completely and the calls with the params property containing an empty array. In both cases, the eventual JSON-RPC request won't contain the params property.

Unfortunately there are services that require messages with empty params, as this behavior does not contradict the official JSON-RPC specification and its handling is at the server's discretion.

Proposed solution

If JsonRpcClient::CallMethod is invoked with two arguments: ID and the method name, generate a request without the params property. If JsonRpcClient::CallMethod is invoked with three arguments and the params argument is an empty vector, then generate a request with the params property containing an empty array.

[cinemast]

Thanks!