Fix OIT analytical shadows crash

[lilleyse]

Fixes #6873

@lb999255 could you check if this fixes the crash you saw?

To do:

• Not really sure how to write a test for this yet. fromLightSource=false isn't tested in general.
• CHANGES.md

[cesium-concierge]

Thanks for the pull request @lilleyse!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.

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[lilleyse]

Updated. Added test for fromLightSource=false that fails in master and passes here.

@bagnell can you review?

[bagnell]

The code changes look good to me. I can confirm that the test fails in master and works in this branch. I'll merge once @lb999255 confirms this fixes the issue.

[flyingdq]

I got the branch and tested on my local project. It's running fine now. Thanks for your help! @lilleyse @bagnell