Account for nullptr root tile in Tileset::loadMetadata

[j9liu]

Sometimes getRootTileAvailableEvent will resolve with a nullptr root tile if there are errors while loading (e.g., attempting to load a tileset from an invalid URL). This is desired behavior, even if the name is slightly misleading for the situation.

The loadMetadata function will now account for this possibility to avoid a nullptr deference error.

[j9liu]

Wait I'm pretty confused. I noticed that the promise is supposed to reject if something goes wrong loading the root tile:

cesium-native/Cesium3DTilesSelection/src/TilesetContentManager.cpp

Line 870 in dfb9c3f

thiz->_rootTileAvailablePromise.reject(

Wouldn't that also make the corresponding future reject? So why wouldn't the getRootTileAvailableEvent reject as well?