BUG: RangeIndex concatenating incorrectly a single object of length 1
- [x] closes #39401
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

@jreback All green! Good to merge? thanks @AnnaDaglis very nice. keep em coming!