Don't create value/image on optional signature field

I have a content type with an optional signature field. I noticed that an image or field value is created when I save the content, regardless of whether or not I actually sign. Is there a way to only generate the image (or create a field value) if someone has marked in the signature field?

My use case is that I want to create a view that shows which nodes have been signed and which have not. Because all nodes have field values for signatures, that's impossible at this point.