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
```