OSM buildings + columbus view crashes

[Sandcastle](https://sandcastle.cesium.com/#c=bZDLTsMwEEV/xcoqkSKnbCGtENkiFYkKNt449tCO8CMaP6qC+HfiRkgU6t1cnzMzdpbEMsIRiK2ZgyMbIGCy/OWc1aJS53rwLkp0QKJq2adwjEUgmpMn8hk10O2PqAhkhFdPRu8WpG5a4b6aO+GEW0bxoMABnwgtRswQuNS6vmiwDfYhodHo9qFuinyhWk/TYecHb5IdUyjb1iu+KlzVVn2IJwObsmY592gnT5ElMjXnXQQ7mXlE6Mak3iFyFUIRC9p3v9VeY2ao11d+gSkjQ5hv3pIxz/gBotr03cz/U42X5RXbDGTkqWCHm83jEnLO+24ur5vRezNK+tP5Gw). It will also crash for the [official OSM buildings sandcastle](https://sandcastle.cesium.com/?src=Cesium%20OSM%20Buildings.html) if you switch to columbus view manually. Also, it might only crash at large enough screen resolutions.

The crash is caused by a bounding sphere with center `0,0,0` being fed into `ellipsoid.cartesianToCartographic`. The bounding sphere is computed in `ModelUtility.computeBoundingSphere` and doesn't update the `min` and `max` bounds, resulting in a bounding sphere with center `0,0,0` and radius `Infinity`. @lilleyse thinks the dummy root tile is responsible.