asgi: check raw_path is not None (#719)

The ASGI spec
(https://asgi.readthedocs.io/en/latest/specs/www.html#http) seems to
imply that `None` is a valid value, so we need to check the value
itself, not just whether the key is present.

In particular, the [mangum](https://github.com/erm/mangum) adapter
passes `None` for this key.

datasette/utils/asgi.py

@@ -46,7 +46,7 @@ def host(self):
 
     @property
     def path(self):
-        if "raw_path" in self.scope:
+        if self.scope.get("raw_path") is not None:
             return self.scope["raw_path"].decode("latin-1")
         else:
             path = self.scope["path"]