fix(dataset): handle plot for dataset with incorrect parameter data

[einsmein]

This PR modifies plot_dataset to handle dataset with incorrect parameter data.

When dataset is somehow exported incorrectly, such that get_parameter_data shows setpoints and dependent params as

{
  "dep": {"dep": [...], "index": [...]}, 
  "setpoint": {"setpoint": [...], "index": [...]}, 
}

instead of

{"dep" {"dep": [...], "setpoint": [...]}}

plot_dataset function previously throws a KeyError.
(Specifically, we see this when a dataset with unsorted setpoint was exported to netcdf with flag use_multi_index="always".)

[jenshnielsen]

Looks good. Perhaps we should add a changelog entry

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 70.60%. Comparing base (5c75dd6) to head (3f359ba).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
src/qcodes/dataset/data_export.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8080   +/-   ##
=======================================
  Coverage   70.59%   70.60%           
=======================================
  Files         333      333           
  Lines       32494    32502    +8     
=======================================
+ Hits        22940    22947    +7     
- Misses       9554     9555    +1     

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