title: Pandas count different combinations of 2 columns with nan
body: <p>I have a dataframe similar to </p>
  
  <pre><code>df = pd.DataFrame({'A': [1, np.nan,2,3, np.nan,4], 'B': [np.nan, 1,np.nan,2, 3, np.nan]})
  df
       A    B
  0  1.0  NaN
  1  NaN  1.0
  2  2.0  NaN
  3  3.0  2.0
  4  NaN  3.0
  5  4.0  NaN
  </code></pre>
  
  <p>How do I count the number of occurrences of A is <code>np.nan</code> but B not <code>np.nan</code>, A not <code>np.nan</code> but B is <code>np.nan</code>, and A and B both not <code>np.nan</code>?</p>
  
  <p>I tried <code>df.groupby(['A', 'B']).count()</code> but it doesn't read the rows with <code>np.nan</code>.</p>