Pandas error when saving dummy data in feather format

[evansd]

I haven't looked into this in any detail, but I opened a random GitHub Actions log for a random project just to copy some example log output and spotted this:

     2022-12-19 09:18:33 [info     ] cohortextractor-stats          [cohortextractor.log_utils] description=generate_cohort execution_time=0:00:00.159524 execution_time_secs=0.1595239189999802 index_date=all state=error study_definition=study_definition_controlfinal time=679.542627243 timing=stop timing_id=0
     Exception at 2022-12-19 09:18:33 UTC
     Traceback (most recent call last):
       File "/opt/venv/bin/cohortextractor", line 8, in <module>
         sys.exit(main())
       File "/app/cohortextractor/cohortextractor.py", line 891, in main
         generate_cohort(
       File "/app/cohortextractor/cohortextractor.py", line 167, in generate_cohort
         _generate_cohort(
       File "/app/cohortextractor/cohortextractor.py", line 240, in _generate_cohort
         study.to_file(
       File "/app/cohortextractor/study_definition.py", line 114, in to_file
         dataframe_to_file(df, filename)
       File "/app/cohortextractor/pandas_utils.py", line 17, in dataframe_to_file
         df.to_feather(filename, compression="zstd")
       File "/opt/venv/lib/python3.8/site-packages/pandas/util/_decorators.py", line 207, in wrapper
         return func(*args, **kwargs)
       File "/opt/venv/lib/python3.8/site-packages/pandas/core/frame.py", line 2681, in to_feather
         to_feather(self, path, **kwargs)
       File "/opt/venv/lib/python3.8/site-packages/pandas/io/feather_format.py", line 67, in to_feather
         raise ValueError(
     ValueError: feather does not support serializing <class 'pandas.core.indexes.base.Index'> for the index; you can .reset_index() to make the index into column(s)

That's from:
https://github.com/opensafely/covid-vaccine-effectiveness-seqtrial/actions/runs/3729963891/jobs/6326466775#step:3:6251

Which was running this action:
https://github.com/opensafely/covid-vaccine-effectiveness-seqtrial/blob/11c1d8381b1d3b3ffef643429315baa2df2de13b/project.yaml#L308-L317

With this version of Cohort Extractor:
https://github.com/opensafely-core/cohort-extractor/pkgs/container/cohortextractor/57403067?tag=1.80.10