BUG: Series.asof fails for all NaN Series (GH15713)
 - [x] closes bug #15713
 - [x] 1 tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry

Added the test if the series is all nans
Added the code that check if that's the case: if yes, return the expected output
As this is my first contribution, please comment if I did it right :)
Thanks!
