HepMC3FileReader: fix loading of run info for RootTree HepMC3 input f…

…ormat

This is done by actually reading an event instead of skipping one.
Skipping may be sufficient for the ASCII reader, however, RootTree reader does not perform a read on skip:
https://gitlab.cern.ch/hepmc/HepMC3/-/blob/master/rootIO/src/ReaderRootTree.cc#L76-80

DDG4/hepmc/HepMC3FileReader.cpp

@@ -130,7 +130,8 @@ HEPMC3FileReader::HEPMC3FileReader(const std::string& nam)
   m_reader = HepMC3::deduce_reader(nam);
 #if HEPMC3_VERSION_CODE >= 3002006
   // to get the runInfo in the Ascii reader we have to force HepMC to read the first event
-  m_reader->skip(1);
+  HepMC3::GenEvent dummy;
+  m_reader->read_event(dummy);
   // then we get the run info (shared pointer)
   auto runInfo = m_reader->run_info();
   // and deallocate the reader