TST: Groupby.filter dropna=False with empty group (#10780)
 - [x] closes #10780
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``

Didn't find a PR in 0.20 or 0.19.2 that closed this, but works on master 0.19.1
## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14926?src=pr) is 84.65% (diff: 100%)
> Merging [#14926](https://codecov.io/gh/pandas-dev/pandas/pull/14926?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #14926   diff @@
==========================================
  Files           144        144          
  Lines         51023      51023          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43190      43191     +1   
+ Misses         7833       7832     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [3ab0e55...ee4ce0f](https://codecov.io/gh/pandas-dev/pandas/compare/3ab0e552c9bac85048dc001d764bb7b903f73a36...ee4ce0f8087436e903497076ea6137521d1e177d?src=pr) Added test for `dropna=True` and all green.  thanks!