Fix root path requests (#5815)

* Fix root path requests

Since #5759 we've tried to access the path explicitly. However, this
raises KeyError exception when trying to access the proxied root path
(e.g. http://supervisor/core/api/). Before #5759 get was used, which
lead to no exception, but instead inserted a `None` into the path.

It seems aiohttp doesn't provide a path when the root is accessed. So
simply convert this to no path as well by setting path to an empty
string.

* Add rudimentary pytest for regular proxy requets

supervisor/api/proxy.py

@@ -117,7 +117,7 @@ class APIProxy(CoreSysAttributes):
             raise HTTPBadGateway()
 
         # Normal request
-        path = request.match_info["path"]
+        path = request.match_info.get("path", "")
         async with self._api_client(request, path) as client:
             data = await client.read()
             return web.Response(