Auto merge of #23021 - ejmg:assert_fail_#18439, r=jdm

Assert fail #18439

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This PR addresses #18439 by removing an assert statement that forces a panic whenever `LoadBlocker` is dropped during a GC sweep and receives a `None` `SCRIPT_THREAD_ROOT` value from `mark_document_with_no_blocked_loads()`. Instead of panicking on the assert, we remove it and let the None value pass silently.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #18439

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