title: NaN is not recognized in pandas after np.where clause. Why? Or is it a bug?
body: <p>NaN is not recognized in pandas after np.where clause. Why? Or is it a bug?</p>
  
  <p>The last line of this code should be "True"</p>
  
  <pre><code>In [1]: import pandas as pd
  In [2]: import numpy as np   
  In [3]: a=pd.Series([1,np.nan])    
  In [4]: b=pd.DataFrame(["a","b"])  
  In [5]: b["1"]=np.where(
              a.isnull(),
              np.nan,
              "Hello"
          )   
  In [6]: b
  Out[6]:
     0      1
  0  a  Hello
  1  b    nan    
  In [7]: b[1].isnull()
  Out[7]:
  0    False
  1    False
  Name: 1, dtype: bool
  </code></pre>