[RF] Fix generating pulls in RooMCStudy.

- [ROOT-10242] RooMCStudy crashes when pulls for a parameter that cannot
have a pull distribution are requested. This adds logic to generate a
proper pull distribution, and returns a blank plot if no pull
distribution can be generated.
- [ROOT-8777] Also fix a rogue cout in RooMCStudy.
- Update the tutorial rf802 to demonstrate how pulls are computed when
generator model and fit model differ.