ACTS TF: protect against eta=nan

• Protect against η=nan in branch stopper, seed KF prefit, and track selector.
  • η=nan can come about if track θ<0 or θ>π.
  • Occasionally occurred in the CKF branch stopper for ttbar_pu200 with flags.Acts.reverseTrackFindingForStrips=True.
  • Would throw std::invalid_argument{"Eta is outside the abs eta bin edges"} exception in Acts::TrackSelector::EtaBinnedConfig::binIndex(eta).