CLN: move _na_value to DatetimeIndexOpsMixin
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`

Because `PeriodIndex` now shares the same `NaT` repr (#13609), can use the same definition on `DatetimeIndexOpsMixin`

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13997?src=pr) is 85.28% (diff: 100%)

> Merging [#13997](https://codecov.io/gh/pydata/pandas/pull/13997?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #13997   diff @@
==========================================
  Files           139        139          
  Lines         50214      50211     -3   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          42822      42820     -2   
+ Misses         7392       7391     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [a0d05db...41268c3](https://codecov.io/gh/pydata/pandas/compare/a0d05dbf7477df863486d82a0cdd4e3e93023864...41268c38985ff8d084d3c4096022d328779eff16?src=pr)
 @sinhrks Looks good. Can be merged?
 lgtm. go ahead @jorisvandenbossche 
