TST GH27994 Add test for dropping empty list / index for a DataFrame …
…with a non-unique datetime index.

- [ ] closes GH#27994 pandas-dev/pandas/issues/27994
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry
