Fix PreserveText preg_split with $limit null

Deprecated in PHP 8.1
- preg_split(): Passing null to parameter mraspe#3 ($limit) of type int is deprecated
- -1 and 0 had same effect in versions before

Author: glady

src/PhpWord/Element/PreserveText.php

@@ -60,7 +60,7 @@ public function __construct($text = null, $fontStyle = null, $paragraphStyle = n
         $this->paragraphStyle = $this->setNewStyle(new Paragraph(), $paragraphStyle);
 
         $this->text = SharedText::toUTF8($text);
-        $matches = preg_split('/({.*?})/', $this->text, null, PREG_SPLIT_DELIM_CAPTURE | PREG_SPLIT_NO_EMPTY);
+        $matches = preg_split('/({.*?})/', $this->text, -1, PREG_SPLIT_DELIM_CAPTURE | PREG_SPLIT_NO_EMPTY);
         if (isset($matches[0])) {
             $this->text = $matches;
         }