[ROOT-10508] Fix TTreeProcessorMT for unnamed chain as friend

* Retrieves correctly the name of the trees building the chain if the
  chain itself does not has a name. This happens if the default
  constructor for TChain is used and the name of the tree is retrieved
  via the string given to `TChain::AddFile("file.root/treeName")`.
* This does not fix the issue entirely since the FriendInfo needs a
  refactoring. The FriendInfo assumes that in a chain each tree has the
  same name, which could not hold true.