Filter unused runtime nodes

[ptrgags]

This PR fixes a corner case where unused glTF nodes would cause Model to crash at runtime.

ModelSceneGraph keeps an array of runtime nodes that is the same length as the list of nodes in the glTF. However, unused nodes are undefined entries in this array. The loops in configurePipeline and update weren't filtering out this corner case.

This bug was first reported in this forum post, see the thread for an example tileset.

[cesium-concierge]

Thanks for the pull request @ptrgags!

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[ptrgags]

This PR is now ready, @sanjeetsuhag could you review?

[sanjeetsuhag]

Looks good @ptrgags. I tested it by adding empty nodes to the BoxInstanced.gltf file and it fixed the crash. From the screenshot posted on the community forum, seems like it was fixed there too.