Change invoke return type on hub_connection

[luizrsilva]

Hi,

This PR, fix an error when CompletionMessage doesn't have error and returns null on result property.

Log Error

I/flutter ( 8307): (WebSockets transport) data received. message String data of length 44. Content: '{"type":3,"invocationId":"0","result":null}�'.
I/flutter ( 8307): (WebSockets transport) error calling onReceive, error: type 'Null' is not a subtype of type 'Object'
I/flutter ( 8307): (WebSockets transport) socket closed.