[Potential Bug] Senders and Receivers can't be None as specified in docstrings

Hello, 

I think there might be a bug in the node aggregation fn when using GCN. The docstrings state that we can set the senders and receivers to None for graphs that have no edges but due to using the senders and receivers in the node aggregation function, setting them to None causes it to break. I assume this is not intended unless I am missing something.

Thank you.