New glTF importer crashes when objects have no valid name

[gboisse]

So, it seems that cgltf returns nullptr names on objects that have no valid name, which crashes std::string as they do not accept nullptr assignments (which is a really annoying thing if you ask me): c31f5ce (could cause the crash by trying to load the Sponza scene)

This ticket is mostly to track whether you think there's other places where this check should be added?

[maoliver-amd]

Side effect of having multiple sponza scenes, I clearly tested the wrong one.