Set trusted.url.domains default to array()

See https://github.com/simplesamlphp/simplesamlphp/issues/265

config-templates/config.php

@@ -834,6 +834,6 @@ $config = array(
      * Example:
      *   'trusted.url.domains' => array('sp.example.com', 'app.example.com'),
      */
-    'trusted.url.domains' => null,
+    'trusted.url.domains' => array(),
 
 );