Adjust histogram boundaries when min(data)==max(data) in plotting

[xjules]

Issue
If min==max then there is no plot. We expect a single bar of length == ensemble size.

• Add test when min==max for histogram plot

Approach
Adjust boundaries.

(Screenshot of new behavior in GUI if applicable)

• 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 91.02%. Comparing base (ebc4d03) to head (e23d56f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8994      +/-   ##
==========================================
+ Coverage   91.00%   91.02%   +0.01%     
==========================================
  Files         349      349              
  Lines       21641    21644       +3     
==========================================
+ Hits        19695    19701       +6     
+ Misses       1946     1943       -3     

Flag	Coverage Δ
cli-tests	39.17% <0.00%> (-0.02%)	⬇️
gui-tests	72.90% <33.33%> (+0.02%)	⬆️
performance-tests	49.69% <0.00%> (-0.03%)	⬇️
unit-tests	79.72% <100.00%> (+0.02%)	⬆️

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