Crash in Cesium3DTilesetTraversal

On current dev I'm getting a exception in `Cesium3DTilesetTraversal`, introduced with #5254. If I ook at point cloud without zooming in all is okay. But If I zoom in it crashes - maybe if I see last level of tree.

Error message:
```
TypeError: Cannot read property 'insert' of undefined
TypeError: Cannot read property 'insert' of undefined
at loadTile (http://localhost:8080/Source/Scene/Cesium3DTilesetTraversal.js:635:30)
at InternalSkipTraversal.leafHandler (http://localhost:8080/Source/Scene/Cesium3DTilesetTraversal.js:568:21)
at DFS (http://localhost:8080/Source/Scene/Cesium3DTilesetTraversal.js:764:25)
at InternalSkipTraversal.execute (http://localhost:8080/Source/Scene/Cesium3DTilesetTraversal.js:479:9)
at SkipTraversal.getChildren (http://localhost:8080/Source/Scene/Cesium3DTilesetTraversal.js:453:26)
at BFS (http://localhost:8080/Source/Scene/Cesium3DTilesetTraversal.js:788:40)
at SkipTraversal.execute (http://localhost:8080/Source/Scene/Cesium3DTilesetTraversal.js:436:9)
at Object.selectTiles (http://localhost:8080/Source/Scene/Cesium3DTilesetTraversal.js:88:40)
at Cesium3DTileset.update (http://localhost:8080/Source/Scene/Cesium3DTileset.js:1577:34)
at PrimitiveCollection.update (http://localhost:8080/Source/Scene/PrimitiveCollection.js:366:27)
at updatePrimitives (http://localhost:8080/Source/Scene/Scene.js:2421:27)
at executeCommandsInViewport (http://localhost:8080/Source/Scene/Scene.js:2287:13)
```
`tile._requestHeap` is still `undefined` at crashing line.

Start by reproducing the point-cloud crash while zooming toward the last tree level on the current dev build. Inspect Source/Scene/Cesium3DTilesetTraversal.js around loadTile at line 635 and the related Cesium3DTileset.js update path, focusing on the reported undefined tile._requestHeap. Done means the zoom operation no longer throws the reported TypeError.

Written by the indexing model from the issue text.