Previously, one could easily "loose" baked data by undoing. The data was not
actually deleted, but Blender didn't find it anymore. This happened because
`update_existing_bake_caches` was expected to be called after changes to
`NodesModifierData.bakes`. However, this was not called after undo.
It was also not possible to call this function from `blend_read`, because it
required access to the referenced node tree.
Now, the bake cache is lazily created during depsgraph evaluation, which
partially solves the problem. It can still seem that Blender lost data, but that
fixes itself much more quickly after the next evaluation. The remaining issue
should be solved a future patch that keeps the bake cache intact over undo
steps using `foreach_cache`.
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