Fix an issue in the mesh parsing code

[bghgary]

Causes overrideMaterialSideOrientation to be undefined

Fixes #14445

[bjsplat]

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[bjsplat]

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[bjsplat]

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[bjsplat]

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[bghgary]

@RaananW Not sure what's going on with the tests, but I'm getting:

sh: 1: jest: not found

https://dev.azure.com/babylonjs/ContinousIntegration/_build/results?buildId=28518&view=logs&jobId=8cb20a0c-5982-5482-f38d-5691146bd923&j=8cb20a0c-5982-5482-f38d-5691146bd923&t=f80791e9-75a6-5a26-3d3b-e313cdebe1dc

[bjsplat]

Visualization tests for webgl2 have failed. If some tests failed because the snapshots do not match, the report can be found at

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[bjsplat]

Visualization tests for webgl1 have failed. If some tests failed because the snapshots do not match, the report can be found at

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