BUG: Series/DataFrame with IntervalIndex doesn't display NaN in index
- [x] closes #25984
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25993?src=pr&el=h1) Report
> Merging [#25993](https://codecov.io/gh/pandas-dev/pandas/pull/25993?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/013f4b4e41f309b839be1de39ab42815af447050?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25993/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25993?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25993      +/-   ##
==========================================
- Coverage   91.86%   91.85%   -0.01%     
==========================================
  Files         175      175              
  Lines       52547    52547              
==========================================
- Hits        48271    48266       -5     
- Misses       4276     4281       +5
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.41% <100%> (ø)` | :arrow_up: |
| #single | `41.9% <100%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25993?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/25993/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9pbnRlcnZhbC5weQ==) | `95.25% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/25993/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `75% <0%> (-12.5%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/25993/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.79% <0%> (-0.12%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25993/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.62% <0%> (-0.11%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25993?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/25993?src=pr&el=footer). Last update [013f4b4...71b8805](https://codecov.io/gh/pandas-dev/pandas/pull/25993?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm. i restarted the build, ping on green. green thanks @simonjayhawkins 