TST:Test to_sparse with nan dataframe (#10079)
 - [x] closes #10079
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``

Possibly fixed with a PR that fixed one of #10627. Look like `to_dense` is fixed for 0.20.0, but should be unrelated to this. 
## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14913?src=pr) is 84.57% (diff: 100%)
> Merging [#14913](https://codecov.io/gh/pandas-dev/pandas/pull/14913?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #14913   diff @@
==========================================
  Files           144        144          
  Lines         51057      51057          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43180      43181     +1   
+ Misses         7877       7876     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [f1cfe5b...48cd7e2](https://codecov.io/gh/pandas-dev/pandas/compare/f1cfe5b4eb614f9b4e201e764e9bd25f2389b355...48cd7e2388471e9229fb32e504b0abb121de8f28?src=pr) thanks @mroeschke !

finding and testing these orphan issues is really great! No problem! Making a push to clean up stale issues that have been solved, so more PRs to come. 