Light linking was never working correctly in volume segment with light
tree, because `sd->object` was not assigned, thus
`light_link_receiver_nee(kg, sd)` always returned `OBJECT_NONE`, causing
the light tree sample to fail. This problem was revealed by fdc2962beb
since now the same light is used for volume segment and volume.
Also ensure we don't sample position on the light if sampling from
volume segment is failed, by setting `emitter_id` to `EMITTER_NONE` in
such cases.
Pull Request: https://projects.blender.org/blender/blender/pulls/122999
f9a75d1e40