Fix key error on empty summary observations in plotter

[yngve-sk]

Issue
Resolves #9560

[codspeed-hq]

CodSpeed Performance Report

Merging #9568 will not alter performance

_{Comparing yngve-sk:24.12.16.bugfix2-emptyensembleresponses (571ca2c) with main (0e19012)}

Summary

✅ 22 untouched benchmarks

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.85%. Comparing base (0d10c5d) to head (571ca2c).
Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9568      +/-   ##
==========================================
- Coverage   91.87%   91.85%   -0.02%     
==========================================
  Files         433      433              
  Lines       26768    26770       +2     
==========================================
- Hits        24593    24590       -3     
- Misses       2175     2180       +5     

Flag	Coverage Δ
cli-tests	39.76% <ø> (-0.03%)	⬇️
everest-models-test	34.58% <ø> (-0.09%)	⬇️
gui-tests	72.14% <ø> (+0.01%)	⬆️
integration-test	37.20% <ø> (-0.02%)	⬇️
performance-tests	51.94% <ø> (-0.02%)	⬇️
test	40.47% <ø> (+<0.01%)	⬆️
unit-tests	74.18% <ø> (+<0.01%)	⬆️

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