Image::sourceSize is being set to 0 when source is an empty string

The current use-case of Image::sourceSize is to tell the Image element that the image should be loaded at a certain resolution. This doesn't match nicely with setting it to 0 when source gets set to an empty string, since that overrides the value it is expected to have, and destroys the effect of sourceSize the next time the source property gets set.

This behavior was introduced in b4bf4f754ba65da3ea751dd5508d29244a4b4db2, and is not seen in 4.7.0.