Send an empty buffer, only for the tag

Property definitions

mpi $ MPI :: send_empty
	# Send an empty buffer, only for the `tag`
	fun send_empty(dest: Rank, tag: Tag, comm: Comm): SuccessOrError
	`{
		return MPI_Send(NULL, 0, MPI_CHAR, dest, tag, comm);
	`}
lib/mpi/mpi.nit:126,2--130,3