_concat_rangeindex_... now returns an empty RangeIndex for empty ranges
This is a follow up to PR #18191.

Overall the code is more readable by adding the constructors at the beginning and
explicitly calling `RangeIndex` constructor.

One range index test was changed to accommodate for the new behavior of the index concatenation function.

- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] 1 test changed


Hello @SmokinCaterpillar! Thanks for updating the PR.





Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on November 11, 2017 at 11:32 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18214?src=pr&el=h1) Report
> Merging [#18214](https://codecov.io/gh/pandas-dev/pandas/pull/18214?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/75c1fa01ebcfe3a61cb1ec9899729062336724a6?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18214/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/18214?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18214      +/-   ##
==========================================
- Coverage   91.42%   91.41%   -0.02%     
==========================================
  Files         163      163              
  Lines       50068    50064       -4     
==========================================
- Hits        45777    45764      -13     
- Misses       4291     4300       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.21% <100%> (-0.01%)` | :arrow_down: |
| #single | `40.36% <33.33%> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18214?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/dtypes/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/18214?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbmNhdC5weQ==) | `99.13% <100%> (-0.02%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18214?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18214?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |

------

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> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18214?src=pr&el=footer). Last update [75c1fa0...3210504](https://codecov.io/gh/pandas-dev/pandas/pull/18214?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!