BUG: Empty DataFrame(..., dtype='datetime64[ns]').quantile(..., axis=1) returns float64 dtype
- [ ] xref #41544 
- [ ] tests added / passed
- [ ] 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
