Merge pull request #2272 from seamuslee001/php81_locale

PHP8.1 Fix issue with passing NULL value for locale into string function

src/PhpWord/Style/Language.php

@@ -232,7 +232,7 @@ private function validateLocale($locale)
             $locale = str_replace('_', '-', $locale);
         }
 
-        if (strlen($locale) === 2) {
+        if ($locale !== null && strlen($locale) === 2) {
             return strtolower($locale) . '-' . strtoupper($locale);
         }