4d0936c7 explicitly avoids turning non-geometry object
instances into geometry instances. This code is called to prepare
geometry sets for baking, and baking currently assumes that the baked
instances are always geometry sets.

To fix this, just check the instance type and serialize an empty
geometry set for the crashing object instance case. Compared to before
the crash causing commit, there is no change in behavior, since that
would have created empty geometry sets too.

Pull Request: https://projects.blender.org/blender/blender/pulls/119892