BUG: IntervalIndex.union with mismatched dtypes both empty
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Ever so slowly ironing out the wrinkles between the setops.
technically needs a whatsnew