`WsgiToAsgiInstance.build_environ` crashes when `scope.server` is present, but set to None

[graysonchao]

Some ASGI servers, such as uvicorn, will sometimes pass a scope object to WsgiToAsgi where scope["server"] is set to None. This causes the app to crash when build_environ tries to access scope["server"] like a tuple (here).

However, extending the check to include that scope["server"] is not None would be an imperfect fix, because it would set SERVER_NAME and SERVER_PORT to "localhost:80" when in some cases that's not true - for example, when launching uvicorn bound to a Unix socket. This latter issue likely needs to be fixed in the server code, though.