str_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated

I finally was able to find a fix for when the subject has no value:

    $params = [
      'private_message' => $message,
      'private_message_thread' => $thread,
      'subject' => str_replace('[site:name]', $site_name, $message_notification_mail_map['subject'] ?? ''),
    ]; 

and yet no emails are sent.

I am now left with a bunch of image errors,

image	03/05/2025 - 17:33	Source image at public://pictures/2025-02/1728477031623…	
image	03/05/2025 - 17:33	Source image at public://pictures/2025-02/IMG_8559.jpeg…	
image	03/05/2025 - 17:33	Source image at public://pictures/2025-02/…	
image	03/05/2025 - 17:33	Source image at public://pictures/2025-02/FABIAN PIZ (1…	
image	03/05/2025 - 17:33	Source image at public://pictures/2025-02/1728477031623…	
image	03/05/2025 - 17:33	Source image at public://pictures/2025-02/…	
image	03/05/2025 - 17:33	Source image at public://pictures/2025-02/IMG_8559.jpeg…	
image	03/05/2025 - 17:33	Source image at public://pictures/2025-02/IMG-20230705-…	
image	03/05/2025 - 17:33	Source image at public://pictures/2025-02/IMG_2977.jpeg…	
image	03/05/2025 - 17:33	Source image at public://pictures/2025-02/FABIAN PIZ (1…	
image	03/05/2025 - 17:33	Source image at public://pictures/2025-01/IMG_1161.jpeg…	
image	03/05/2025 - 17:33	Source image at public://pictures/2025-02/IMG_2022-08-…

Opening one of them looks like this,

Source image at public://pictures/2025-02/1728477031623.jpg.webp not found while trying to generate derivative image at public://styles/thumbnail/public/pictures/2025-02/1728477031623.jpg.webp.

I moved to live server, updated message, message_notify, and private_message modules. I think I applied a few more patches, and now some emails are delivering. The emails don't deliver consistently cos I did set to deliver emails 'For every private message' but it is not delivery for 'every' message so far.

At least it is comforting that some emails are delivering. I will keep watching the pattern.