BUG: Mitigate division with zero in roll_var
- [ ] closes #42442 
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

cc @mroeschke  @benHeid 

> However, I am unsure about writing tests for pandas since this is my first PR to pandas. I would appreciate help here. Thank you.

Take a look at 

https://pandas.pydata.org/pandas-docs/stable/development/contributing_codebase.html#testing-with-continuous-integration

as well as the tests in `pandas.tests.window`, and feel free to ask questions here. Hello @benHeid! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2021-07-24 11:12:40 UTC I added a parameterized test with different min_periods and windows. there are still some precommit issues here @github-actions pre-commit
 can you add a release note in 1.4 bug fix section (under rolling) thanks @benHeid 