PHP 8.1: fix various deprecations/errors in newest version of PHP (#310)

* Test on PHP 8.1

* PHP 8.1: fix deprecated NULL param to glob()

* PHP 8.1: fix PHP error when passing NULL to rawurlencode()

* PHP 8.1: calling ctype_lower with FALSE is deprecated

* PHP 8.1: passing NULL to setAttribute() is deprecated

* PHP 8.1: passing NULL to str_replace() is an error

* PHP 8.1: fix error passing NULL to str_replace()

* PHP 8.1: fix return type deprecation with backwards compatible attribute

* Revert typo

library/HTMLPurifier/StringHash.php

@@ -20,6 +20,7 @@ class HTMLPurifier_StringHash extends ArrayObject
      * @param mixed $index
      * @return mixed
      */
+    #[\ReturnTypeWillChange]
     public function offsetGet($index)
     {
         $this->accessed[$index] = true;