fix installer. fixes #5

The installer sets an empty array config cascade which messed with our
preload.

root/var/www/html/inc/preload.php

@@ -9,6 +9,8 @@
 // load the standard cascade if the farmer hasn't already loaded it
 if (!defined('DOKU_CONF')) {
     define('DOKU_CONF', __DIR__ . '/../conf/');
+}
+if (!isset($config_cascade) || $config_cascade === []) {
     include __DIR__ . '/config_cascade.php';
 }