status/network: Use NM connectivity check URI for Portal Helper

NetworkManager can check if a network interface has "internet access" by
invoking a HTTP request.

The URI used for connectivity checking in NetworkManager can be configured
manually in NetworkManager.conf:

[connectivity]
uri=http://portal-check.exmaple.com/nm-check.txt

Portal Helper provides an argument to pass the URI that should be opened.
If this argument is empty it uses http://nmcheck.gnome.org as a fallback.

Pass the URI configured in NetworkManager to Portal Helper instead of
an empty string.

Fixes #1313

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2228>

js/ui/status/network.js

@@ -2060,7 +2060,7 @@ class Indicator extends SystemIndicator {
             }
         }
 
-        this._portalHelperProxy?.AuthenticateAsync(path, '', timestamp).catch(logError);
+        this._portalHelperProxy?.AuthenticateAsync(path, this._client.connectivity_check_uri, timestamp).catch(logError);
 
         this._connectivityQueue.add(path);
     }