fix: add explicit nullable type for additionalQuery parameter

Add explicit nullable type declaration for $additionalQuery parameter in findBySlug method to address PHP 8.4 deprecation warning

src/HasSlug.php

@@ -190,7 +190,7 @@ protected function generateSubstring($slugSourceString)
         return substr($slugSourceString, 0, $this->slugOptions->maximumLength);
     }
 
-    public static function findBySlug(string $slug, array $columns = ['*'], callable $additionalQuery = null)
+    public static function findBySlug(string $slug, array $columns = ['*'], ?callable $additionalQuery = null)
     {
         $modelInstance = new static();
         $field = $modelInstance->getSlugOptions()->slugField;