T2345: Walking back nullptr changes in #defines in STUBS/mpi.h

#define MPI_GROUP_EMPTY -1

#define MPI_ANY_SOURCE -1

#define MPI_STATUS_IGNORE nullptr

#define MPI_STATUS_IGNORE NULL

#define MPI_Comm int

#define MPI_Request int

#define MPI_Group int

#define MPI_Offset long

#define MPI_IN_PLACE nullptr

#define MPI_IN_PLACE NULL

#define MPI_MAX_PROCESSOR_NAME 128

#define MPI_MAX_LIBRARY_VERSION_STRING 128