Use iens = 0 to avoid error with ensemble_size=1

[eivindjahren]

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'pytest tests/ert/unit_tests -n logical -m "not integration_test"')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Create Backport PR to latest release

[berland]

lgtm!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.36%. Comparing base (321767c) to head (a03d62c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8817      +/-   ##
==========================================
- Coverage   91.36%   91.36%   -0.01%     
==========================================
  Files         342      342              
  Lines       20995    20995              
==========================================
- Hits        19183    19182       -1     
- Misses       1812     1813       +1     

Flag	Coverage Δ
cli-tests	39.62% <ø> (+<0.01%)	⬆️
gui-tests	73.53% <ø> (+0.02%)	⬆️
performance-tests	50.08% <ø> (ø)
unit-tests	79.83% <ø> (-0.01%)	⬇️

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