monitor-manager: Let the backends handle no configuration

Let the backends decide whether to just rebuild a derived state, or use
the NULL config to rebuild an empty logical state.

This also changes the expected screen size values of the no-outputs
test; as this case is actually handled now.

https://bugzilla.gnome.org/show_bug.cgi?id=777732

src/backends/meta-monitor-manager.c

@@ -420,7 +420,7 @@ done:
 
   if (!config)
     {
-      meta_monitor_manager_rebuild (manager, NULL);
+      meta_monitor_manager_apply_monitors_config (manager, NULL, &error);
       return NULL;
     }