Add None check for grpc.aio interceptor

[ordinary-jamie]

Add a None check in gRPC AIO interceptor.

Address issue: #3108

grpc.aio.ServerInterceptor should expect the continuation to return None if the RPC isn't handled. Checking the return of continuation is not None before getting the handler_factory prevents an AttributeError

References

1. gRPC Python Docs on return types for interceptors, https://grpc.github.io/grpc/python/grpc_asyncio.html#grpc.aio.ServerInterceptor
2. Example implementation: https://github.com/d5h-foss/grpc-interceptor/blob/master/src/grpc_interceptor/server.py#L48-L57

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[sl0thentr0py]

ty @ordinary-jamie !
need to make the linter happy, see comment