Supress warning on absent NameIDFormat

After updating to 1.17 I started noticing a lot of warnings in the log:
\Mar  4 12:47:16 sv1810934 IDP-BZK[5791]: 3 [c11b0ed290] Unable to generate NameID. Check the userid.attribute option.

SP doesn't ask for a specific NameIDFormat and none is set in the metadata...
current([]) would return false, but we never catch that case,..

modules/saml/lib/IdP/SAML2.php

@@ -1206,6 +1206,9 @@ private static function buildAssertion(
             $nameIdFormat = current($spMetadata->getArrayizeString('NameIDFormat', []));
             if ($nameIdFormat === null) {
                 $nameIdFormat = current($idpMetadata->getArrayizeString('NameIDFormat', [\SAML2\Constants::NAMEID_TRANSIENT]));
+            } elseif ($nameIdFormat === false) {
+                // Not set in request or in metadata. Fall back to transient
+                $nameIdFormat = \SAML2\Constants::NAMEID_TRANSIENT;
             }
         }