Default value to getArrayizeString must also be array if we want to prevent PHP notices

modules/saml/lib/IdP/SAML2.php

@@ -1004,9 +1004,9 @@ class SAML2
 
         if ($nameIdFormat === null || !isset($state['saml:NameID'][$nameIdFormat])) {
             // either not set in request, or not set to a format we supply. Fall back to old generation method
-            $nameIdFormat = current($spMetadata->getArrayizeString('NameIDFormat', null));
+            $nameIdFormat = current($spMetadata->getArrayizeString('NameIDFormat', []));
             if ($nameIdFormat === null) {
-                $nameIdFormat = current($idpMetadata->getArrayizeString('NameIDFormat', \SAML2\Constants::NAMEID_TRANSIENT));
+                $nameIdFormat = current($idpMetadata->getArrayizeString('NameIDFormat', [\SAML2\Constants::NAMEID_TRANSIENT]));
             }
         }