message: Fix parameter to C++ Material wrapper, must be empty str not null
diff: @@ -214,7 +214,7 @@ cpp::Instance makeLocalSpheres(
  sphereGeom.commit();
  vec3f color(0.f, 0.f, (mpiRank + 1.f) / mpiWorldSize);
  - cpp::Material material(nullptr, "obj");
  + cpp::Material material("", "obj");
  material.setParam("kd", color);
  material.commit();
  