Fix crash due to terrain loading race condition

[ggetz]

Follow up from #11296

Technically after that change, due to this line, it was possible to return a promise which resolves to undefined if a requestTileGeometry request gets throttled by the requestScheduler.

[cesium-concierge]

Thanks for the pull request @ggetz!

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  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.

[jjhembd]

Thanks @ggetz!