图片不存在时，如果 src 本身是个 ReactElement，会尝试回退到 src，否则尝试回退到 icon，最后回退到显示 children。
src
icon
children
When the image does not exist, if src is a ReactElement, it will try to fall back to src, otherwise it will try to fall back to icon, and finally fall back to displaying children.
