ERR: disallow RangeIndex()
xref #13749 

this is not consistent with other Indexes where we don't allow them to be constructed with no data passed (it can be `None`), but that's the idea. `start` must be not-None.

IOW this should raise `ValueError`

```
In [3]: pd.RangeIndex()
Out[3]: RangeIndex(start=0, stop=0, step=1)
```
