Fix tile refinement when leaf is empty

[lilleyse]

We got a support issue about a tileset that doesn't refine property when skipLevelOfDetail is false, which is the default since CesiumJS 1.67. The tileset has empty leaf tiles which exposed an edge case in executeEmptyTraversal in Cesium3DTilesetTraversal in which a tile with empty leaf descendants would not refine. The fix is to refine even if the tile would refine to empty leaf tiles. Note that this only applies to leaf tiles - if there's a tile chain that consists of a content tile, an empty tile, and a content tile, it will wait for the final content tile to load before refining the top-level content tile, which is the main purpose of executeEmptyTraversal- to prevent holes from appearing while tiles stream in.

Technically this fix could lead to situations where the parent will refine and there will be holes wherever there's empty leaf tiles, but I think that's acceptable and in the spirit of how the tileset is constructed.

Local sandcastle with the fix
Hosted sancastle

[cesium-concierge]

Thanks for the pull request @lilleyse!

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[OmarShehata]

Forum user confirmed this branch fixes their 3D Tileset generated by Context Capture: https://community.cesium.com/t/3dtiles-are-only-partially-displayed-with-skiplevelofdetail-disabled/10066/5?u=omar

[mramato]

@loshjawrence Can you please take a look at this?

[loshjawrence]

Looks ok, waiting for ci to pass.