BUG: Align to_csv signatures with DataFrame and Series
Default `path=None` in `Series.to_csv()` for consistency with `DataFrame.to_csv()`.

Closes #14054.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14063?src=pr) is 85.27% (diff: 100%)

> Merging [#14063](https://codecov.io/gh/pydata/pandas/pull/14063?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #14063   diff @@
==========================================
  Files           139        139          
  Lines         50446      50446          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43018      43018          
  Misses         7428       7428          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [df2d9ab...389ae8c](https://codecov.io/gh/pydata/pandas/compare/df2d9ab917a1ef4732cfb6c3317432f60c071074...389ae8c4149a4c026d99cdbb51b4228b287d11da?src=pr)
 thanks!
