Avoid UserWarning when vmin==vmax in std_dev plot

[berland]

Issue
Resolves #9657

Approach
if

• 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 ✅

Project coverage is 91.71%. Comparing base (839610b) to head (f433ef8).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9659      +/-   ##
==========================================
- Coverage   91.74%   91.71%   -0.03%     
==========================================
  Files         430      430              
  Lines       26639    26640       +1     
==========================================
- Hits        24439    24434       -5     
- Misses       2200     2206       +6     

Flag	Coverage Δ
cli-tests	39.69% <0.00%> (-0.01%)	⬇️
everest-models-test	34.18% <0.00%> (-0.01%)	⬇️
gui-tests	71.83% <100.00%> (-0.03%)	⬇️
integration-test	38.29% <0.00%> (-0.01%)	⬇️
performance-tests	51.68% <0.00%> (-0.01%)	⬇️
test	39.62% <0.00%> (+<0.01%)	⬆️
unit-tests	73.99% <100.00%> (+<0.01%)	⬆️

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[codspeed-hq]

CodSpeed Performance Report

Merging #9659 will not alter performance

_{Comparing berland:avoid_userwarning (f433ef8) with main (7d73f74)}

Summary

✅ 24 untouched benchmarks