Layer reports true on `HasImage`, panics when encoded as image

[drdreo]

Behavior
When a certain layer is a vector (simple pen tool shape), HasImage() returns true but the Picker of said layer has no image data which panics on png.Encode with the message:
png: invalid format: invalid image size: 0x0

Expected
A layer which currently can not be parsed, and hence, has no image data, should return false on HasImage.

Code

        if !l.HasImage() {
		return nil
	}
	fmt.Printf("%s -> %s.png\n", layerName, filename)
	out, err := os.Create(fmt.Sprintf("%s.png", filename))
	if err != nil {
		return err
	}
	defer out.Close()
	return png.Encode(out, l.Picker)