helper: Port Comm MPI backend away from SMPI_ wrappers
helper: Port Comm MPI backend away from SMPI_ wrappers

If `CommFromMPI()` is given `MPI_COMM_NULL`, fall back to `CommDummy()`
instead.

ornladios/ADIOS2,JasonRuonanWang/ADIOS2,JasonRuonanWang/ADIOS2,ornladios/ADIOS2,ornladios/ADIOS2,JasonRuonanWang/ADIOS2,ornladios/ADIOS2,ornladios/ADIOS2,JasonRuonanWang/ADIOS2,JasonRuonanWang/ADIOS2