BUG: PeriodIndex.min/max returns int
Related to #7279. `PeriodIndex.min/max` should return `Period` ignoring `NaT`.

```
pidx = pd.PeriodIndex(['2011-01', 'NaT'], freq='M')
pidx.min()
# -9223372036854775808
pidx.max()
#492
```
