Decal: Add support for thin instances

[Popov72]

See https://forum.babylonjs.com/t/thin-instance-decals/40759

This PR also suppresses the "Setting vertex data kind 'XXX' with an empty array" warning when the created mesh is empty.

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

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