Work around std::vector SFINAE problem

[dsarmany]

Description

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

🌈🌦️📖🚧 Documentation 🚧📖🌦️🌈
https://sites.ecmwf.int/docs/dev-section/multio/pull-requests/PR-232

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.88%. Comparing base (9c7bdd0) to head (97d65dc).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #232      +/-   ##
===========================================
- Coverage    57.88%   57.88%   -0.01%     
===========================================
  Files          319      319              
  Lines        20533    20533              
  Branches      1595     1593       -2     
===========================================
- Hits         11886    11885       -1     
- Misses        8647     8648       +1     

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