use the empty string instead of null as an array offset

Author: xabbuh

Slugger/AsciiSlugger.php

@@ -131,8 +131,8 @@ 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])) {