How to generate PASCAL VOC xml for pure-background images?

What if, there are some images not contains an object, just pure-background. How can generate PASCAL VOC xml file too? in order to use it in deep learning training for object detection. I mean, the same images in training folder should be also have the same xml file in annotation folder, whether it contains an object or not. If anyone have idea, could you tell me how to solve it. Thanks in advance.

here, an example of xml file for pure-background and one object in the image.

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