Fix bug in window function count should count anything non-null
 - [x] closes #12541 
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry

this looks right, pls add a note in whatsnew, bug fix section.

note that I did try to change this a while back and got some tests failures (which I think you mentioned), so these need to be investigated / fixed. I think the 'floatifying' is slightly different that checking for non-null, but need to drill down and see exactly. ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/15196?src=pr) is 85.98% (diff: 100%)
> Merging [#15196](https://codecov.io/gh/pandas-dev/pandas/pull/15196?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #15196   diff @@
==========================================
  Files           140        140          
  Lines         51263      51267     +4   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          44076      44084     +8   
+ Misses         7187       7183     -4   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [be3f2ae...65d70eb](https://codecov.io/gh/pandas-dev/pandas/compare/be3f2aea4a8ad3f6bf4eb84e6ca3269abff5fcfb...65d70eb614d8b799f5c155768448763ceaa64e26?src=pr) lgtm. ping on green (I will fix the minor typo on merge) thanks @mralgos 