[stable33] fix: Subject in encrypt method

[SystemKeeper]

Before (subject empty)

Data to encrypt is: {"nid":163098,"app":"admin_notifications","subject":"","type":"admin_notifications","id":"6a393593"}

After

Data to encrypt is: {"nid":163099,"app":"admin_notifications","subject":"Testing push notifications","type":"admin_notifications","id":"6a3935bd"}

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI

[nickvergessen]

/backport! to stable32