monitor-config-manager: Handle headless setup gracefully

Handle headless setup gracefully by having no logical monitors. This
commit only makes the monitor management code deal with it; other areas
may still not be able to handle it.

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

src/backends/native/meta-monitor-manager-kms.c

@@ -1499,6 +1499,13 @@ meta_monitor_manager_kms_apply_monitors_config (MetaMonitorManager *manager,
   GPtrArray *crtc_infos;
   GPtrArray *output_infos;
 
+  if (!config)
+    {
+      manager->screen_width = 0;
+      manager->screen_height = 0;
+      return TRUE;
+    }
+
   if (!meta_monitor_config_manager_assign (manager, config,
                                            &crtc_infos, &output_infos,
                                            error))