BUG: fix str.contains for series containing only nan values
- [x] closes #14171
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14182?src=pr) is 85.24% (diff: 100%)

> Merging [#14182](https://codecov.io/gh/pydata/pandas/pull/14182?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #14182   diff @@
==========================================
  Files           140        140          
  Lines         50549      50550     +1   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43090      43091     +1   
  Misses         7459       7459          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [d8cd33b...c7e9721](https://codecov.io/gh/pydata/pandas/compare/d8cd33b148daba78f1450c19c69cfe4896cfb98c...c7e972105b605be7c1f6928cc520cdc059ea081e?src=pr)
 thanks! nicely done
