BUG: Rolling returned nan with FixedForwardWindowIndexer for count when window contained only missing values
- [x] closes #35579
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Issue still has milestone 1.1.3. If it's to late for this release, I'll move the whats new note to 1.1.4 for when the associated PR is merged.

cc @mroeschke 
Have just seen #36649 which will close the issue.