Fix for 'idpdisco.extDiscoveryStorage is not a valid string value in discopower

See old issue: https://code.google.com/p/simplesamlphp/issues/detail?id=613

modules/discopower/lib/PowerIdPDisco.php

@@ -190,7 +190,7 @@ class sspmod_discopower_PowerIdPDisco extends SimpleSAML_XHTML_IdPDisco {
 		$idp = $this->getTargetIdp();
 		if($idp !== NULL) {
 
-			$extDiscoveryStorage = $this->config->getString('idpdisco.extDiscoveryStorage');
+			$extDiscoveryStorage = $this->config->getString('idpdisco.extDiscoveryStorage',NULL);
 			if ($extDiscoveryStorage !== NULL) {
 				$this->log('Choice made [' . $idp . '] (Forwarding to external discovery storage)');
 				SimpleSAML_Utilities::redirectTrustedURL($extDiscoveryStorage, array(