In theme_emimage_image() why are height and width required?

In the theme function to render images, theme_emimage_image(), parameters 6 and 7 are image width and height. The very next block of code checks to see if they are defined, and if not tries to set some appropriate values.

I think both of these should have a default value of NULL. This would allow the function to be called with one of those parameters empty, and have the image resized appropriately. The attached patch makes the very small change.