Why does SW Header Mega keep the XL image when Desktop Image is empty?

XL is the image used for the largest screens, and it is also the fallback for smaller screen sizes. If Desktop Image is empty, the module keeps the XL image and resizes it for the desktop layout. If Tablet Image is also empty, the same fallback continues to tablet. The order is XL image, then an optional Desktop override, then an optional Tablet override; leaving an override empty does not hide the image.