Change the versioning code so that all bone groups are converted to bone
collections, so also the ones that did not have any bones assigned.
As Demeter[1] put it: While bone groups with 0 bones assigned are
usually unintended, versioning should still preserve them I feel like,
just to be on the safe side. If there was an update to the vertex group
system, I would also expect empty vertex groups to survive, even though
they are strictly speaking pretty much pointless.
[1]: https://projects.blender.org/blender/blender/issues/111711#issuecomment-1013159
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