auth_fc: disable authenticator if improperly configured (#43653)

src/authentic2_auth_fc/authenticators.py

@@ -28,7 +28,9 @@ class FcAuthenticator(BaseAuthenticator):
     id = 'fc'
 
     def enabled(self):
-        return app_settings.enable
+        return (app_settings.enable and
+                app_settings.client_id and
+                app_settings.client_secret)
 
     def name(self):
         return gettext_noop('FranceConnect')