Handle missing light-link values

[nicolaspopravka]

Compatibility:

patch

Issues/Tickets:

nicolaspopravka/usd-render-benchmark#23

Release notes comment:

Handle missing light-link and shadow-link values without reporting coding errors.

Comments for the reviewer:

HdSceneDelegate::GetLightParamValue() may return an empty VtValue when the optional lightLink or shadowLink parameter is absent.

Light::Sync() currently calls Get<TfToken>() unconditionally for both parameters. This change checks that each value contains a TfToken and otherwise uses an empty category.

Testing:

• An OpenMoonRay v2026.29.1 / OpenUSD 25.05.01 build completed its stock usdrecord test without the empty-token or invalid-framebuffer diagnostics: https://github.com/nicolaspopravka/aswf-docker/actions/runs/31696672171
• A CY2025 MoonRay benchmark rerun produced none of those diagnostics in the McUsd, OpenChessSet, or ALab logs. OpenChessSet completed cleanly. ALab encountered a separate intermittent texture-read problem tracked in ALab intermittently renders with missing textures from RunPod network volume nicolaspopravka/usd-render-benchmark#27.

Look or scene setup change:

Special notes for production:

Attention/Reviewers:

AI Assisted Development:

Assisted-by: OpenAI Codex / GPT-5

Checklist:

• Documentation has been updated.
• Includes new unit tests.
• Includes new RATS tests.