BUG: PeriodIndex and Period subtraction error
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Similar to #5202. `PeriodIndex` subtraction raises `AttributeError` either side is `Period` (scalar).

```
pd.PeriodIndex(['2011-01', '2011-02'], freq='M') - pd.Period('2011-01', freq='M') 
# AttributeError: 'PeriodIndex' object has no attribute 'ordinal'
```
#### Expected

If `Period(NaT)` is included in either side, result is `Float64Index` to hold `nan`.

```
pd.PeriodIndex(['2011-01', '2011-02'], freq='M') - pd.Period('2011-01', freq='M') 
# Int64Index([0, 1], dtype='int64')

pd.PeriodIndex(['2011-01', 'NaT'], freq='M') - pd.Period('2011-01', freq='M') 
Float64Index([0.0, nan], dtype='float64')
```
