BUG: Fix IntervalIndex.to_tuples() with NA values
- [X] closes #18756
- [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/18757?src=pr&el=h1) Report
> Merging [#18757](https://codecov.io/gh/pandas-dev/pandas/pull/18757?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7b4229aa1b95b2050b31cc93e38fda1a72258675?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18757/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18757?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18757      +/-   ##
==========================================
- Coverage   91.63%   91.59%   -0.05%     
==========================================
  Files         154      154              
  Lines       51419    51422       +3     
==========================================
- Hits        47120    47102      -18     
- Misses       4299     4320      +21
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.46% <100%> (-0.03%)` | :arrow_down: |
| #single | `40.82% <20%> (-0.12%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18757?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/18757/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9pbnRlcnZhbC5weQ==) | `93.83% <100%> (+0.03%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18757/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18757/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `64.78% <0%> (-1.74%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18757/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.68% <0%> (-0.11%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18757?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/18757?src=pr&el=footer). Last update [7b4229a...7c330c6](https://codecov.io/gh/pandas-dev/pandas/pull/18757?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Added the `na_tuple` kwarg.  Had to do some explicit checks of the NA value in the test, due equality issues with tuples of NA (see https://github.com/pandas-dev/pandas/issues/18756#issuecomment-351881631 for an example) thanks @jschendel  Indeed, thanks!