When loading an unknown node type from a newer Blender version, the node
storage data cannot be properly loaded. Re-saving the file will then
crash if saving with the same node type idname, since new Blender
versions cannot find the expected storage data.
Loading in older versions should replace unknown node types with a dummy
"Undefined" node that will get loaded as NodeTypeUndefined in newer
versions as well. Custom node types are exempted from this since they
store all data as generic ID properties and can always be fully
serialized.
This is a revised version of the initial attempt in #114803.
Doing the node type fix in the after-linking stage ensures that
versioning code can change outdated node types which might otherwise get
removed by this type check.
Pull Request: https://projects.blender.org/blender/blender/pulls/116908
7981b35064