Previously we often passed an empty `FunctionRef` to the generic
`search_gather` function. The plan is to refactor `search_gather`
to return an `IndexMask` along with storing leaf nodes in a separate
array. Splitting access to all leaf nodes to a separate function simplifies
this process and makes code more expressive too.
Part of #118145.
Pull Request: https://projects.blender.org/blender/blender/pulls/126667
d5e591b3dc