Fix intermittent clamp to ground failure

[hpinkos]

Fixes #6858

If the tile isn't finished loading, tile.data is the upsampled data from the parent tile, which isn't the correct height for that LOD.

[cesium-concierge]

Thanks for the pull request @hpinkos!

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[hpinkos]

@bagnell it's easy to reproduce the issue in master if you use the examle in #6858 and follow these steps:

• Run the example
• Once the globe is loaded, open the network tab in the debugger and throttle the download speed (i created a custom throttle profile with 4048 kb/s download speed)
• Click the 'Set View To Entities' button while

[bagnell]

👍