Some document have non-standard locale code

Author: Ernestas Staugaitis

src/PhpWord/Style/Language.php

@@ -228,7 +228,9 @@ public function getBidirectional()
      */
     private function validateLocale($locale)
     {
-        $locale = str_replace('_', '-', $locale);
+        if ($locale !== null) {
+            $locale = str_replace('_', '-', $locale);
+        }
 
         if (strlen($locale) === 2) {
             return strtolower($locale) . '-' . strtoupper($locale);