[ntuple] Fix-up variant field following RFieldBase::Create() changes
[ntuple] Fix-up variant field following RFieldBase::Create() changes

olifre/root,root-mirror/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,root-mirror/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root,root-mirror/root