Use count() instead of empty().

Cannot provide array_filter() output directly to empty() in conditional in PHP <= 5.4.

lib/SimpleSAML/Utils/Config/Metadata.php

@@ -117,7 +117,7 @@ class Metadata
 
         // check attributes is an associative array
         if (isset($contact['attributes'])) {
-            if (empty($contact['attributes']) || !is_array($contact['attributes']) || empty(array_filter(array_keys($contact['attributes']), 'is_string'))) {
+            if (empty($contact['attributes']) || !is_array($contact['attributes']) || count(array_filter(array_keys($contact['attributes']), 'is_string')) == 0) {
                 throw new \InvalidArgumentException('"attributes" must be an array and cannot be empty.');
             }
         }