mpi: Prevent errors with single rank #8480

Job	Run time
pytest-ubuntu-py312-gcc12-cxxomp-base	52m 22s
pytest-ubuntu-py312-gcc12-cxxomp-adjoint	36m 43s
pytest-ubuntu-py311-gcc11-cxxnoomp-base	41m 48s
pytest-ubuntu-py310-gcc9-omp-adjoint	30m 19s
pytest-ubuntu-py312-gcc13-omp-adjoint	34m 37s
pytest-ubuntu-py312-gcc13-omp-base	49m 23s
pytest-ubuntu-py310-gcc14-omp-base	41m 58s
pytest-osx-py312-clang-omp-base	26m 17s
pytest-ubuntu-py311-gcc10-noomp-base	40m 10s
pytest-ubuntu-py311-gcc11-cxxnoomp-adjoint	30m 3s
pytest-ubuntu-py311-gcc10-noomp-adjoint	28m 31s
pytest-ubuntu-py310-gcc14-omp-adjoint	29m 57s
pytest-ubuntu-py313-gcc14-omp-base	49m 36s
pytest-docker-py310-icx-omp-adjoint	25m 36s
pytest-ubuntu-py310-gcc9-omp-base	41m 22s
pytest-docker-py310-icx-omp-base	28m 50s
pytest-docker-py310-gcc-omp-base	35m 5s
pytest-ubuntu-py313-gcc14-omp-adjoint	36m 21s
pytest-docker-py310-gcc-omp-adjoint	26m 40s
	11h 25m 38s

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