Update src/User.php

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: spam-n-eggs

src/User.php

@@ -9,7 +9,7 @@ class User implements Arrayable
     public function __construct(
         public ?string $userId = null,
         public ?string $anonymousId = null,
-        public ?array $additionalFields = [],
+        public array $additionalFields = [],
         public ?array $traits = [],
     ) {}