return empty df for measureddata w/o responses

[yngve-sk]

Issue
Resolves failing (one but maybe not all) semeio tests

Approach
Handle case of missing responses

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 90.85%. Comparing base (64bbc7e) to head (abe2c52).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ert/data/_measured_data.py	40.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9044      +/-   ##
==========================================
- Coverage   90.93%   90.85%   -0.08%     
==========================================
  Files         352      352              
  Lines       21712    21751      +39     
==========================================
+ Hits        19743    19762      +19     
- Misses       1969     1989      +20     

Flag	Coverage Δ
cli-tests	39.01% <0.00%> (-0.07%)	⬇️
gui-tests	72.54% <40.00%> (-0.13%)	⬇️
performance-tests	49.46% <0.00%> (-0.09%)	⬇️
unit-tests	79.60% <40.00%> (-0.07%)	⬇️

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