message: use nullptr for pointer initialization
diff: @@ -362,7 +362,7 @@ void generate_multidomain_blueprint(sidre::DataStore* ds,
  for(int64_t i = domain_begin; i < domain_end; ++i)
  {
  - sidre::Group* mroot = 0;
  + sidre::Group* mroot = nullptr;
  // The list format does not use string names to identify the domains
  // held by group 'holder`, while the map format in the else block
  