BUG: RangeIndex can be created without args
- [x] closes #13793
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

lgtm.
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13803?src=pr) is 85.23% (diff: 100%)

> Merging [#13803](https://codecov.io/gh/pydata/pandas/pull/13803?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #13803   diff @@
==========================================
  Files           140        140          
  Lines         50415      50420     +5   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          42970      42974     +4   
- Misses         7445       7446     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [98c5b88...aab6ae6](https://codecov.io/gh/pydata/pandas/compare/98c5b88d6f9f7bb0afa4fbae49a045d93a1cb33f...aab6ae6bc39f881dcaf93ed5df2d3513c3168880?src=pr)
 thanks @sinhrks 
