TST/BUG: Added more tests for Period(NaT)
- [x] follow-up of #12759
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry
- because `Period(NaT)` had separate impl until #12579, there were some bugs related to `pd.isnull`. Now all the issues are fixed, thus added explicit tests and what's new. 
- Also updated what's new to describe `Period(None)` (related to #13723).

**0.18.1:**

```
s = pd.Series([pd.Period('2011-01', freq='M'), pd.Period('NaT', freq='M')])
s.isnull()
#0    False
#1    False
# dtype: bool

s.fillna(pd.Period('2012-01', freq='M'))
#0   2011-01
#1       NaT
# dtype: object

s.dropna()
#0   2011-01
#1       NaT
# dtype: object
```
