frontpage: Avoid URL reverse during Shortcut component construction

This allows the entire app construction process not to require Django.

Tests:

- An app with url is None (ejabberd) and one without (transmission) show their
on the frontpage. Both shortcuts work as expected.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/frontpage.py

@@ -70,7 +70,7 @@ class Shortcut(app.FollowerComponent):
         """
         super().__init__(component_id)
 
-        if not url:
+        if url is None:
             url = '?selected={id}'.format(id=component_id)
 
         self.name = name