Inconsistent crash for polygons with empty hierarchy

### Error raised if 1 or more entities has polygon with empty hierarchy array, but not all!!!

**Sandcastle example**: https://cesiumjs.org/Cesium/Apps/Sandcastle/index.html#c=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

**Browser**: any

**Operating System**: any

Start with the linked Cesium Sandcastle example and reproduce the crash using entities whose polygons have empty hierarchy arrays, varying whether all entities or only some are affected. Trace the polygon hierarchy handling from that entry point; done means mixed empty and non-empty hierarchies no longer crash and behavior is consistent.

Written by the indexing model from the issue text.