Setting style of `ModelExperimental` before it's ready causes an error

[j9liu]

Setting model.style before the readyPromise is resolved leads to this error:

This is because the style setter doesn't check if the model has loaded or not, so it's trying to work with undefined components. It'd be nice if this were consistent with Cesium3DTileset, which correctly applies the style once the tileset is ready.

Sandcastle example