There seems to be some ambiguity in the standard with respect to usage of the MPI_DATATYPE_NULL variable.

It would seem to me that if an MPI routine such as MPI_Send or MPI_ALLTOALLW has a data count argument of 0

then MPI_DATATYPE_NULL should be an acceptable value for the datatype.   Here is a discussion from several years

ago with openmpi developers.  It references a discussion in the standards forum but I can no longer find it.