Make StandardWells support cases with no wells.
Dense case was handled in PR #1002. I was not aware that the same
regression was in the legacy code. wells_manager.c_wells() might return
a null pointer if there are no wells in the domain of a process. In this case
we treat it as having zero wells.
Thanks for all the fixes @blattms, I have not commented much on them, but you should know we appreciate them!