refactor: notification initialization and assignment logic

[appleboy]

• Add checks to initialize req.Notification if req.Title, req.Message, or req.Image are not empty
• Assign req.Title, req.Message, and req.Image to req.Notification fields if they are not empty
• Rename notification variable to message for consistency
• Remove redundant notification initialization and assignment logic
• Ensure message.Data is assigned if req.Data is not empty

ref #773