Throw from XmlReader::expand() if DOM extension missing

Otherwise it is hard to uphold the arginfo contract for this
function -- we cannot simply mirror the zpp call, as the class
entry it is based on does not exist.

ext/xmlreader/php_xmlreader.stub.php

@@ -76,6 +76,6 @@ class XMLReader
     /** @return bool|XMLReader */
     public static function XML(string $source, ?string $encoding = null, int $options = 0) {}
 
-    /** @return DOMNode|false|null */
+    /** @return DOMNode|false */
     public function expand(?DOMNode $basenode = null) {}
 }