v4: Don't try to determine MPI stack if MPI not found

ESMA_cmake itself does not require MPI, so if MPI is not found, we cannot determine an MPI stack. This release fixes that bug.

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From CHANGELOG.md

Fixed

• Do not include DetermineMPIStack if MPI is not found

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What's Changed

• v4: Do not determine MPI stack without MPI found by @mathomp4 in #410

Full Changelog: v4.8.0...v4.8.1