The realize instances code didn't have a way to use the existing
`#BuiltinAttributeProvider::default_value()`s to initialize attributes that
have to be created.
Now this writes default values of builtin attributes to the curve `attribute_fallbacks`.
Removes the need for the code to explicitly write the `resolution` and
`nurbs_weight`.
The other attributes that are written explicitly (like `radius`) don't have
builtin default values unfortunately. Ideally those would also just be provided
by the respective `BuiltinAttributeProvider`.
Pull Request: https://projects.blender.org/blender/blender/pulls/131799
94120967e7