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

[codecov-commenter]

Codecov Report

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

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

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/equinor/ert/pull/8817/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=equinor) | Coverage Δ | | |---|---|---| | [cli-tests](https://app.codecov.io/gh/equinor/ert/pull/8817/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=equinor) | `39.62% <ø> (+<0.01%)` | :arrow_up: | | [gui-tests](https://app.codecov.io/gh/equinor/ert/pull/8817/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=equinor) | `73.53% <ø> (+0.02%)` | :arrow_up: | | [performance-tests](https://app.codecov.io/gh/equinor/ert/pull/8817/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=equinor) | `50.08% <ø> (ø)` | | | [unit-tests](https://app.codecov.io/gh/equinor/ert/pull/8817/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=equinor) | `79.83% <ø> (-0.01%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=equinor#carryforward-flags-in-the-pull-request-comment) to find out more.

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