TST/BUG: Added more tests for Period(NaT)

[sinhrks]

• follow-up of API: Change Period('NAT') to return NaT, not Period instance #12759
• tests added / passed
• passes git diff upstream/master | flake8 --diff
• whatsnew entry
• because Period(NaT) had separate impl until DOC: clean up some doc-build warnings #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 BUG: Fix pd.Timedelta(None) to return NaT. #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

[jorisvandenbossche]

Looks good to me!

[jreback]

thanks!