Fix loading models with some but not all Draco compression

[bagnell]

Fix models crashing where some primitive attributes were draco compressed and others were not.
Fixes #7383.

There was an attempt to add de-quantization to the shader, but returned undefined. Any attempt to use the shader string later would cause a crash.

[cesium-concierge]

Thanks for the pull request @bagnell!

• ✔️ Signed CLA found.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[lilleyse]

Thanks @bagnell.