Skip Python loading for C++ shader plugins

[nicolaspopravka]

Compatibility:

patch

Issues/Tickets:

nicolaspopravka/usd-render-benchmark#26

Release notes comment:

Avoid warnings about missing Python modules when loading the MoonRay shader discovery and parser plugins.

Comments for the reviewer:

These libraries provide C++ plugins but no Python bindings. Registering an empty Python module token prevents TfScriptModuleLoader from trying to import nonexistent pxr.MoonrayShaderDiscovery and pxr.MoonrayShaderParser modules.

I tested the change by building OpenMoonRay v2026.29.1 with the patched shader plugins. Shader and renderer discovery remained available, both missing-module warnings were absent, and the MoonRay, hd_render, and stock usdrecord render checks produced varying images.

Validation run: https://github.com/nicolaspopravka/aswf-docker/actions/runs/31791756348

Look or scene setup change:

No expected look change.

Special notes for production:

Attention/Reviewers:

AI Assisted Development:

Assisted-by: OpenAI Codex / GPT-5

I used Codex to help investigate and implement this change and draft the initial description. I reviewed the code and validation results and take responsibility for the contribution.

Checklist:

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