isnull not working for object series with NaT

[cancan101]

In [694]:
series = pd.Series([pd.NaT, '1'])
series[~pd.isnull(series)]

Out[694]:
0    NaN
1      1
dtype: object

Possibly related: #5967 (comment)

[jreback]

On master this works

(aside from the display issue)

Pls feel free to create a test for this though

In [5]: s = pd.Series([pd.NaT,'1'])

In [6]: s
Out[6]: 
0    NaN
1      1
dtype: object

In [7]: pd.isnull(s)
Out[7]: 
0     True
1    False
dtype: bool

In [8]: s[pd.isnull(s)]
Out[8]: 
0   NaT
dtype: datetime64[ns]

In [9]: s[~pd.isnull(s)]
Out[9]: 
1    1
dtype: object

[jreback]

this was closed by #5443