Thanks @axel.
Everything make sense then. Summary, in case it helps somebody out there:
The header file for std::vector (stl_vector.h) in gcc 4.8.5 defines a vector<T>::data() function
that dereferences nullptr in case of an empty vector. This data() method is called in PyROOT when
iterating over the vector. This caused an exception in cling for versions earlier than 6.14/06 on
systems where the headers come from gcc 4.8.