refactor: refactor sound request handling and validation

- Change `req.Sound` type check from string to interface
- Add type assertion for `req.Sound` to string
- Ensure `req.APNS` and `req.Android` are only set if `req.Sound` is a string

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

notify/notification_fcm.go

@@ -74,21 +74,23 @@ func GetAndroidNotification(req *PushNotification) []*messaging.Message {
 	}
 
 	// Check if the notification has a sound
-	if req.Sound != "" {
-		if req.APNS == nil {
+	if req.Sound != nil {
+		sound, ok := req.Sound.(string)
+
+		if req.APNS == nil && ok {
 			req.APNS = &messaging.APNSConfig{
 				Payload: &messaging.APNSPayload{
 					Aps: &messaging.Aps{
-						Sound: req.Sound.(string),
+						Sound: sound,
 					},
 				},
 			}
 		}
 
-		if req.Android == nil {
+		if req.Android == nil && ok {
 			req.Android = &messaging.AndroidConfig{
 				Notification: &messaging.AndroidNotification{
-					Sound: req.Sound.(string),
+					Sound: sound,
 				},
 			}
 		}