_Period if self is NaT
https://github.com/pandas-dev/pandas/blob/master/pandas/_libs/period.pyx#L805

```
cdef class _Period(object):
   [...]
   def __sub__(self, other):
       if isinstance(self, Period):
       [...]
       elif isinstance(other, Period):
           if self is NaT:
               return NaT
       [...]
```

1) Is the `self is NaT` branch possible?

2) Are there any non-trivial cases where the `isinstance(self, Period)` is Falsey?