[DF] SaveGraph is confused by Define nodes in certain cases

~ root                                                                                                                                                                                                         (py3) 

root [0] ROOT::RDataFrame df(10)

(ROOT::RDataFrame &) An empty data frame that will create 10 entries

root [1] auto df2 = df.Define("x", "1")

(ROOT::RDF::RInterface<ROOT::Detail::RDF::RLoopManager, void> &) @0x7fc713204060

root [2] ROOT::RDF::SaveGraph(df2)

digraph {

    0 [label="10", style="filled", fillcolor="#e8f8fc", shape="oval"];

}

root [3]