[RF] RooDataSet::sumEntries not callable from Python.

Due to a missing converter in the old cppyy, one cannot call a function
that takes a `const char*` using a nullptr.
To make RooDataSet::sumEntries callable from Python, it now also takes
empty strings.

See also:
https://root-forum.cern.ch/t/sumentries-cutrange-in-pyroot/33499