Fix and test TimedeltaIndex.__rfloordiv__ bug
I thought we were ready to get rid of _TimeOp, but doing so turned up a couple of new bugs.

Setup:
```
scalar_td = pd.Timedelta('10m7s')
tdi = TimedeltaIndex(['00:05:03', '00:05:03', pd.NaT], freq=None)
```

Before:
```
>>> tdi.__rfloordiv__(scalar_td)
Float64Index([0.0, 0.0, nan], dtype='float64')
```

After:
```
>>> tdi.__rfloordiv__(scalar_td)
Float64Index([2.0, 2.0, nan], dtype='float64')
```

- [ ] closes #xxxx
- [x] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19125?src=pr&el=h1) Report
> Merging [#19125](https://codecov.io/gh/pandas-dev/pandas/pull/19125?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/1265f66860a1a108b835881ef41ca4e52d397b3f?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19125/graphs/tree.svg?width=650&height=150&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/19125?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19125      +/-   ##
==========================================
+ Coverage   91.51%   91.53%   +0.02%     
==========================================
  Files         148      148              
  Lines       48777    48780       +3     
==========================================
+ Hits        44636    44651      +15     
+ Misses       4141     4129      -12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.9% <100%> (+0.02%)` | :arrow_up: |
| #single | `41.59% <25%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19125?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/19125/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.45% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexes/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/19125/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy90aW1lZGVsdGFzLnB5) | `90.63% <100%> (+0.05%)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19125/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `66.95% <0%> (+1.73%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19125?src=pr&el=continue).
> **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/19125?src=pr&el=footer). Last update [1265f66...d76237b](https://codecov.io/gh/pandas-dev/pandas/pull/19125?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!