Image variants without either a width or height are not scaled properly

When an image is created with a variant which has a width or height of 0, the image is saved as the original.

For example when the thumbnail is configured as follows: height: 60, upscaling: false, width: 0. The created thumbnail will have the same sizes as the original image.

It seems that the calculateResizeRatio of the ScaleImageOperation isn't returning the correct ratio in this case.