mpi: Avoid UnboundLocalVariable error

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 86.81%. Comparing base (38c1709) to head (93acdf6).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2386 +/- ## ======================================= Coverage 86.81% 86.81% ======================================= Files 234 234 Lines 44004 44004 Branches 8127 8127 ======================================= Hits 38204 38204 Misses 5093 5093 Partials 707 707 ```

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[FabioLuporini]

Not sure what the error was here

me neither honestly

somehow triggered by PRO, maybe due to the latest Numpy 2.0 release maybe not, hard to say, but yeah