Don't try to paint an image layer with null image.
Don't try to paint an image layer with null image.

I hemmed and hawed over whether to leave this triggering a null pointer
exception (which will crash your app on Android and iOS). In theory nothing
good can come of having an imageless ImageLayer in your scene graph. I could
refuse to add an ImageLayer with no image to the scene graph, but that could be
annoying if you know the image will get set later, before the layer is to be
rendered.

In any case, crashing your app with a useless stack trace (it doesn't tell you
where this image layer was added) is certainly not the right solution. So we'll
just silently acquiesce for the time being.

KoriSamui/PlayN,KoriSamui/PlayN,KoriSamui/PlayN,KoriSamui/PlayN