Merge branch '6.4' into 7.3

* 6.4:
  fix test setup
  [Validator] Review Turkish translations
  [Validator] Review Croatian translations
  [Validator] Review translations for Polish (pl)
  use the empty string instead of null as an array offset
  Review translations for Chinese (zh_TW)
  [Serializer] Adjust ObjectNormalizerTest for the accessor method changes from #61097

Author: xabbuh

Slugger/AsciiSlugger.php

@@ -128,8 +128,8 @@ public function slug(string $string, string $separator = '-', ?string $locale =
 
         if (\is_array($this->symbolsMap)) {
             $map = null;
-            if (isset($this->symbolsMap[$locale])) {
-                $map = $this->symbolsMap[$locale];
+            if (isset($this->symbolsMap[$locale ?? ''])) {
+                $map = $this->symbolsMap[$locale ?? ''];
             } else {
                 $parent = self::getParentLocale($locale);
                 if ($parent && isset($this->symbolsMap[$parent])) {