Fix+Test Timedelta.__mul__(nan)
Specifically, `np.float64("NAN")` which in master gets caught by the `hasattr(other, "dtype")` branch and incorrectly returns `np.timedelta64("NaT", "ns")` instead of `NaT`

- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
