Properly handle prefix = null

Author: tvdijen

src/XPath/XPath.php

@@ -125,7 +125,7 @@ private static function registerAncestorNamespaces(Dom\XPath $xp, Dom\Node $node
                     $uri = (string) $attr->nodeValue;
 
                     if (
-                        $prefix === '' || $prefix === 'xmlns' || $uri === '' || isset($prefixToUri[$prefix])
+                        $prefix === '' || $prefix === null || $prefix === 'xmlns' || $uri === '' || isset($prefixToUri[$prefix])
                     ) {
                         continue;
                     }