nullish element is passed for treemap data.onover/out callback function

[netil]

Steps to check or reproduce

{
  data: {
    type: "treemap",
    onover(d, element) {
	console.log("onover", d, element)
        // --> element is null
    },
    onout(d, element) {
	console.log("onout", d, element)
        // --> element is null
    }
}