BUG: fixes indexing with monotonic decreasing DTI (#19362)
- [x] closes #19362
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Seems that #19362 is caused by the fact that `np.any([0])` is false. Here's a suggested solution. Also added a test.