[bug fix] issue #1168 by adding eval_null_transmission to trampoline class

[DanielPSchenk]

[bug fix] for issue #1168

Description

This small contribution fixes an issue I had as suggested by @merlinND.

Fixes #1168

Testing

I verified that eval_null_transmission is called as expected after the fix and that my python BSDF (see #1168) behaves exactly the same as the reference C++ plugin.

Checklist

• [x] My code follows the style guidelines of this project
• [x] My changes generate no new warnings
• [x] My code also compiles for cuda_* and llvm_* variants. If you can't test this, please leave below
• [x] I have commented my code: not necessary
• [x] I have made corresponding changes to the documentation: not necessary
• [ ] I have added tests that prove my fix is effective or that my feature works: I verified that it fixes the problem I had
• [x] I cleaned the commit history and removed any "Merge" commits
• [x] I give permission that the Mitsuba 3 project may redistribute my contributions under the terms of its license