REF: avoid special case in DTA/TDA.median, flesh out tests
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Also fixes currently-incorrect empty case.
can you rebase Rebased + greenish thanks