BUG: DataFrame.to_markdown with an empty frame
- [x] closes #31771
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Picking up #36233. 

Bumping the minimum version of tabulate as per https://github.com/pandas-dev/pandas/issues/31771#issuecomment-685401053
thanks @arw2019 