calendar-server: Update views only after the time range is set

The gnome-shell-calendar-server calls to refresh queries even when it
has no time range set, which results in:
  a) waste of resources (for example after login),
  b) many runtime warnings in the journalctl log, related to
     incorrect time range being used.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/244

src/calendar-server/gnome-shell-calendar-server.c

@@ -590,6 +590,11 @@ app_load_events (App *app)
   g_list_free (app->live_views);
   app->live_views = NULL;
 
+  if (!app->since || !app->until)
+    {
+      print_debug ("Skipping load of events, no time interval set yet");
+      return;
+    }
   /* timezone could have changed */
   app_update_timezone (app);