Allow calling MPI_Init without arguments
This is a modification of !1332 (merged) where no error is thorwn if MPIHelper::instance() is called without argument. This is a valid initialization for MPI and thus should be also for MPIHelper.
This is a modification of !1332 (merged) where no error is thorwn if MPIHelper::instance() is called without argument. This is a valid initialization for MPI and thus should be also for MPIHelper.