In case the friend tree/chain has not been added manually after
reading the TTree, but it comes automatically from information saved
to file, chain.GetListOfFriends() returns a nullptr while
chain.GetTree()->GetListOfFriends() correctly returns that TTree's list
of friends as per the information on file.
More discussion at https://github.com/root-project/root/issues/6741.

This fixes the RDF part of #6741.