title: Python pandas .isnull() does not work on NaT in object dtype
body: <p>I have this series:</p>
  
  <pre><code>ser=pd.Series([11,22,33,np.nan,np.datetime64('nat')],name='my_series')
  </code></pre>
  
  <p>The series looks like this:</p>
  
  <pre><code>0     11
  1     22
  2     33
  3    NaN
  4    NaN
  Name: my_series, dtype: object
  </code></pre>
  
  <p>But I get only one <code>True</code> for NULL values:</p>
  
  <pre><code>ser.isnull()
  
  0    False
  1    False
  2    False
  3     True
  4    False
  Name: my_series, dtype: bool
  </code></pre>
  
  <p>Is it a bug or how can I count correctly the NULL values in a pandas series? This does not help:</p>
  
  <pre><code>ser=ser.replace('NaN',np.nan)
  </code></pre>
  
  <p>Thanks!</p>