Graphics created with instancing have a null range.

[matmarchand]

Describe the bug
Some type of graphic will not produce a valid range when using instancing. I tested PointString which was fine but not Solid(Sphere) and Gltf.

To Reproduce
Steps to reproduce the behavior:

1. Checkout branch : https://github.com/iTwin/itwinjs-core/tree/MathieuM/GraphicInstancesRange
2. Go to \itwinjs-core\core\frontend
3. npm run test RenderInstances
4. Test will fail

Expected behavior
I expect graphics to have a non null range so I can use it in a TileTree as the root tile.

[pmconne]

@danieliborra @markschlosseratbentley