When using light linking with the light tree, the root index of a
mesh light subtree can be 0. The current code assumed this wasn't
possible, and as such it caused rendering issues, specifically the
incorrect computation of the PDF of certain mesh lights during
forward path tracing.
So we adjust the code to allow mesh light subtree root node
indices of 0.
This was worked on by Alaska, Sergey, and Weizhen
Pull Request: https://projects.blender.org/blender/blender/pulls/119770
43cef92f66