Ensure non-existent gen_kw does not error in dark_storage

[eivindjahren]

Issue
Resolves #8946

Approach
Return empty dataframe when the key does not exist

• 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

[xjules]

small comment only, but otherwise good.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.51%. Comparing base (12d2b53) to head (4713a15).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8956      +/-   ##
==========================================
- Coverage   91.55%   91.51%   -0.04%     
==========================================
  Files         344      344              
  Lines       21295    21296       +1     
==========================================
- Hits        19496    19489       -7     
- Misses       1799     1807       +8     

Flag	Coverage Δ
cli-tests	39.52% <0.00%> (-0.21%)	⬇️
gui-tests	73.52% <0.00%> (-0.04%)	⬇️
performance-tests	50.32% <0.00%> (+0.01%)	⬆️
unit-tests	80.32% <100.00%> (+0.01%)	⬆️

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[xjules]

Nice one! 🚀