BUG: df.to_markdown() with empty frame incorrect output
- [x] closes #31771
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] added test_empty_frame for test_to_markdown.py (#31771)
- [x] Required tabulate >= 0.8.7
