TST: resample does not yield empty groups (#10603)
- [x] closes #10603
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

This adds a test to verify that resample does not yield empty groups.

I'm new to contributing to pandas, and I'm not sure where to add this test.
Let me know if this place is inappropriate.