BUG: fix df.where(cond) when cond is empty
- when cond is empty, cond.dtypes are objects, which raised
`ValueError: Boolean array expected for the condition, not object
`
- [ ] closes #xxxxx 
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
