Fix deprecated null array offset usage in Reader_Partial

Author: lacatoire

phpdotnet/phd/Reader/Partial.php

@@ -31,7 +31,7 @@ public function read(): bool {
         static $arraySkip = array();
 
         while($ret = parent::read()) {
-            $id = $this->getAttributeNs("id", self::XMLNS_XML);
+            $id = $this->getAttributeNs("id", self::XMLNS_XML) ?? '';
             $currentPartial = end($arrayPartial);
             $currentSkip = end($arraySkip);
             if (isset($this->partial[$id])) {