Make RPC params optional

[wiverson]

I have a fair number of functions that rely on auth id for results with no params.

What kind of change does this PR introduce?

Just a minor tweak to make the RPC parameters optional. Shouldn't affect existing code.

What is the current behavior?

Switching the signature from

public Task<BaseResponse> Rpc(string procedureName, Dictionary<string, object> parameters)

to

public Task<BaseResponse> Rpc(string procedureName, Dictionary<string, object>? parameters = null)

[acupofjose]

LGTM!