Receive an empty buffer, only for the tag

Property definitions

mpi $ MPI :: recv_empty
	# Receive an empty buffer, only for the `tag`
	fun recv_empty(dest: Rank, tag: Tag, comm: Comm): SuccessOrError
	`{
		return MPI_Recv(NULL, 0, MPI_CHAR, dest, tag, comm, MPI_STATUS_IGNORE);
	`}
lib/mpi/mpi.nit:132,2--136,3