The main reason that didn't work before because undefined custom node groups
have the undefined `bNodeType` (and thus are ignored in various places) but are
actually still groups that can be evaluated. The fix is just to handle custom
node groups a bit more explicitly.
In the future, we may want to have a separate "undefined custom group"
`bNodetype`, but that might a be a bit bigger project.
Pull Request: https://projects.blender.org/blender/blender/pulls/135974
51904839ac