Fix PHP 8.1 deprecations

Author: derrabus

Slugger/AsciiSlugger.php

@@ -103,7 +103,7 @@ public function slug(string $string, string $separator = '-', string $locale = n
         $locale = $locale ?? $this->defaultLocale;
 
         $transliterator = [];
-        if ('de' === $locale || 0 === strpos($locale, 'de_')) {
+        if ($locale && ('de' === $locale || 0 === strpos($locale, 'de_'))) {
             // Use the shortcut for German in UnicodeString::ascii() if possible (faster and no requirement on intl)
             $transliterator = ['de-ASCII'];
         } elseif (\function_exists('transliterator_transliterate') && $locale) {