ENH: PeriodIndex now accepts pd.NaT
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Related to #12759. `PeirodIndex` can be created from list/array which contains `pd.NaT`.

currently it raises:

```
pd.PeriodIndex([pd.NaT])
# ValueError: freq not specified and cannot be inferred from first element

pd.PeriodIndex([pd.Period('2011-01', freq='M'), pd.NaT])
# AttributeError: 'NaTType' object has no attribute 'ordinal'
```

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13430?src=pr) is **84.25%**

> Merging [#13430](https://codecov.io/gh/pydata/pandas/pull/13430?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #13430   diff @@
==========================================
  Files           138        138          
  Lines         50810      50805     -5   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          42809      42805     -4   
+ Misses         8001       8000     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [07761c5...5990de6](https://codecov.io/gh/pydata/pandas/compare/07761c519154a6ed8a9345512476152226499ba6...5990de6ae425dc6b59495418c8b38daeceaa5285)
 lgtm. ping on green.
 ty sir!
