TST: Removed regex warning for read_csv
Follow-up to #13481 and partially fixes #13932.  Regex mistakenly matches to the empty string, which will cause Python 3.x to issue a warning.  In addition, not sure why this test was placed in `python_parsers_only.py`...

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13943?src=pr) is 85.30% (diff: 100%)

> Merging [#13943](https://codecov.io/gh/pydata/pandas/pull/13943?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #13943   diff @@
==========================================
  Files           139        139          
  Lines         50157      50157          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          42785      42784     -1   
- Misses         7372       7373     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [b7abef4...b93325e](https://codecov.io/gh/pydata/pandas/compare/b7abef4949fb1ba7fd1004feba4f47ace7004282...b93325ee425d473a2e023029810cc077ee31c97c?src=pr)
 ty!
