YOLO label format, when images doesn't have bounding box

If an image (abc.jpg) doesn't have any bounding box, how should look its corresponding label file (abc.txt)?

The regular label format is:

label x_center/width y_center/height width/image_width, height/image_height.

For each yolo version you should check its corresponding dataloader to see how it handles empty images. For Yolov5 I found the following: "if no objects in image, no *.txt file is required". You can find the source for this in their original documentation. Again, this may not be the case for all the other Yolo versions as there is no unified dataloader used by all of the Yolo versions