BUG: rolling mean and sum not numerical stable for all nan window
- [x] closes #41053
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

The problem is not numerical stable, so we are dividing -3e-18 / 0, which returns -inf running into line 177 setting to 0. Checking for nobs > 0 would solve these cases

cc @mroeschke 
cc @mroeschke 
I've added the sum fix mentioned in the issue, the test returns

```
              0
0  2.000000e-03
1  1.000000e-02
2  1.500000e-02
3  1.300000e-02
4  5.000000e-03
5 -8.673617e-19
```

on master. Sum should be the only other case we have the problem. As long as all nan should return 0 we can do it this way thanks @phofl 