Fix error in ASGI middleware if the ASGI client is None

[imjoehaines]

Goal

Our ASGI middleware assumes that the client in the ASGI scope cannot be None, but this isn't the case:

client (Iterable[Unicode string, int]) – A two-item iterable of [host, port], where host is the remote host’s IPv4 or IPv6 address, and port is the remote port as an integer. Optional; if missing defaults to None.
https://asgi.readthedocs.io/en/latest/specs/www.html#http-connection-scope

This PR adds a guard to avoid an error if there is a client but it is None

See #295

Testing

Added a unit test that runs an ASGI app with the client set to None. This fails in master but passes in this branch

I also manually tested with Uvicorn on a unix socket (#295 (comment))