This PR fixes a reported error:

2025/11/18 04:38:47 [error] 2621#2621: *902533 FastCGI sent in stderr: "PHP message: PHP Fatal error: Uncaught TypeError: Beyondwords\Wordpress\Core\Core::getLangCodeFromJsonIfEmpty(): Argument #3 ($meta_key) must be of type string, null given

The issue was that getLangCodeFromJsonIfEmpty() is hooked to WordPress's get_post_metadata filter, but when get_post_meta() is called without specifying a meta key (to fetch all metadata for a post), WordPress passes null as the $meta_key argument.

The fix: Changed the type hint from string $meta_key to ?string $meta_key to allow nullable values.