Handle empty ensemble in observations view

[yngve-sk]

Issue
Resolves #9549

• 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 91.88%. Comparing base (ebe548e) to head (83c5d95).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9554      +/-   ##
==========================================
+ Coverage   91.87%   91.88%   +0.01%     
==========================================
  Files         433      433              
  Lines       26767    26769       +2     
==========================================
+ Hits        24591    24598       +7     
+ Misses       2176     2171       -5     

Flag	Coverage Δ
cli-tests	39.77% <0.00%> (-0.01%)	⬇️
everest-models-test	34.67% <0.00%> (+<0.01%)	⬆️
gui-tests	72.18% <100.00%> (+0.06%)	⬆️
integration-test	37.21% <0.00%> (-0.03%)	⬇️
performance-tests	51.97% <0.00%> (-0.01%)	⬇️
test	40.48% <0.00%> (-0.01%)	⬇️
unit-tests	74.16% <0.00%> (+0.02%)	⬆️

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[codspeed-hq]

CodSpeed Performance Report

Merging #9554 will improve performances by 85.07%

_{Comparing yngve-sk:24.12.16.handle-empty-ens-edgecase-gui (83c5d95) with main (0d10c5d)}

Summary

⚡ 1 improvements
✅ 22 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	yngve-sk:24.12.16.handle-empty-ens-edgecase-gui	Change
⚡	test_gui_snapshot	3.6 s	1.9 s	+85.07%

[eivindjahren]

The regression test does not catch the bug.

[eivindjahren]

👍