status/location: Handle geoclue not being activatable

In GNOME OS, due to a misconfiguration, geoclue was configured without
systemd support. In that configuration, geoclue does not install its
systemd .service file (geoclue.service) but it (incorrectly) includes
the following line in its D-Bus service file:

    SystemdService=geoclue.service

As a result, when dbus-daemon tried to activate it at gnome-shell's
request, it would fail with:

    Unit geoclue.service not found

Then, GeoclueAgent._onGeoclueVanished() would be called, as the
name_vanished_handler passed to Gio.bus_watch_name(). This is consistent
with Gio.bus_watch_name()'s documentation:

> You are guaranteed that one of the handlers will be invoked after
> calling this function.

But that function assumed that this._managerProxy is defined, leading
to:

    JS ERROR: TypeError: this._managerProxy is undefined
    _onGeoclueVanished@resource:///org/gnome/shell/ui/status/location.js:163:9

Fix this by checking for nullishness of this._managerProxy. (Strictly
speaking, it's undefined rather than being null, but other code in this
file already uses the vaguer '!= null' test, which considers undefined
to be null.)

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2689>

js/ui/status/location.js

@@ -160,7 +160,7 @@ var GeoclueAgent = GObject.registerClass({
     }
 
     _onGeoclueVanished() {
-        this._managerProxy.disconnectObject(this);
+        this._managerProxy?.disconnectObject(this);
         this._managerProxy = null;
 
         this.notify('in-use');