Rpc should accept nullable

[ilmalte]

Considering the equivalent unity library:
https://github.com/heroiclabs/nakama-dotnet/blob/f01478cb40b61f5bd8267e61d75856d9e8f4c8ae/Nakama/Client.cs#L1178-L1218

I suppose the methods for rpc should accept nullable session:
686f007

[lugehorsam]

Hey @ilmalte it would be a new method accepting an http key rather than making the session nullable, but yes we would welcome a contribution in this area.