Mock gpu pointcloud_search calls with empty custom attributes

[chellmuth]

Description

The GPU calling convention for variadic shadeops is to pass a pointer to the extra parameters as the op's final argument. Currently calls to osl_pointcloud_search don't follow this convention, so their ptx fails to compile due to argument count mismatches.

This PR doesn't implement the full correct behavior, it just adds a nullptr to the end of OptiX calls to osl_pointcloud_search. This makes everything at least compile, so scenes that have pointcloud operations can fail more gracefully on GPU (for now).

Tests

Checklist:

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have ensured that the change is tested somewhere in the testsuite (adding new test cases if necessary).
• My code follows the prevailing code style of this project. If I haven't
  already run clang-format v17 before submitting, I definitely will look at
  the CI test that runs clang-format and fix anything that it highlights as
  being nonconforming.