Fix MPI_Waitall call in the case of empty processors

This allows to plug the synchronization in ConnectivityDispatcher::_buildNewOwner

src/mesh/ConnectivityDispatcher.cpp

@@ -41,8 +41,7 @@ ConnectivityDispatcher<Dimension>::_buildNewOwner()
         item_new_owner[l] = cell_new_owner[Jmin];
       });
 
-#warning Add missing synchronize (fix it!)
-    // synchronize(face_new_owner);
+    synchronize(item_new_owner);
     this->_dispatchedInfo<item_type>().m_new_owner = item_new_owner;
   }
 }

src/utils/Messenger.hpp

@@ -231,6 +231,7 @@ class Messenger
       }
     }
 
+    if (request_list.size()>0) {
       std::vector<MPI_Status> status_list(request_list.size());
       if (MPI_SUCCESS != MPI_Waitall(request_list.size(), &(request_list[0]), &(status_list[0]))) {
         // LCOV_EXCL_START
@@ -238,6 +239,7 @@ class Messenger
         std::exit(1);
         // LCOV_EXCL_STOP
       }
+    }
 
 #else // PASTIS_HAS_MPI
     Assert(sent_array_list.size() == 1);