Fix y label bug

[eivindjahren]

This should have no effect on normal use of the histogram plot, but some of the defensive programming would just crash.

• [x] PR title captures the intent of the changes, and is fitting for release notes.
• [x] Added appropriate release note label
• [x] Commit history is consistent and clean, in line with the contribution guidelines.
• [x] Make sure tests pass locally (after every commit!)

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

Attention: Patch coverage is 68.00000% with 8 lines in your changes missing coverage. Please review.

Project coverage is 86.04%. Comparing base (9b517d2) to head (fbadd14). Report is 1 commits behind head on main.

Files	Patch %	Lines
src/ert/gui/plottery/plots/histogram.py	68.00%	8 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #8118 +/- ## ========================================== - Coverage 86.07% 86.04% -0.04% ========================================== Files 382 382 Lines 23728 23726 -2 Branches 627 620 -7 ========================================== - Hits 20425 20415 -10 - Misses 3228 3231 +3 - Partials 75 80 +5 ```

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