Check childTileMask if provider doesn't know tile availability.

[kring]

When the terrain tileset doesn't include a layer.json file, the only way to determine tile availability is to look at the parent tile's childTileMask. But I neglected to do that in my terrain refactoring.

Note that the absence of a layer.json means we have no choice to be refine down through the tile pyramid one level at a time, so adding a layer.json is highly recommended for performance reasons. But at least with this PR we won't request a bunch of non-existent tiles.

Fixes #7847

[cesium-concierge]

Thanks for the pull request @kring!

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[OmarShehata]

Thanks a lot @kring ! I've been trying to debug this issue on the forum for a while.

@tfili I can confirm this fixes the issue shown in the Sandcastle in the linked issue. Code looks good. Do you want to do a final review and merge?