Fixing issue with SGI's MPT MPI library

SGI's MPT MPI library is missing the MPI_CONVERSION_FN_NULL
function so add a workaround here. There is a similar fix
in https://gitlab.kitware.com/third-party/mpi4py but it's not clear
now to get that fix into VTK and with Ben Boeckel out I didn't
want to wait for him to come back for this fix to get into VTK.