BUG: If Series is called with no arguments, let it have a RangeIndex (#16737)
 - [x] closes #16737
 - [ ] tests added / passed
 - [x] passes ``git diff upstream/master -u -- "*.py" | flake8 --diff``
 - [x] whatsnew entry

Before:
```python
In [4]: Series().index
Out[4]: Index([], dtype='object')
```

Now:
```python
In [3]: Series().index
Out[3]: RangeIndex(start=0, stop=0, step=1)
```