title: Select data when specific columns have null value in pandas
body: <p>I have a dataframe where there are 2 date fields I want to filter and see rows when any one of the date field is null. </p>
  
  <pre><code>ID          Date1       Date2
  58844880    04/11/16    NaN
  59745846    04/12/16    04/14/16
  59743311    04/13/16    NaN
  59745848    04/14/16    04/11/16
  59598413    NaN         NaN
  59745921    04/14/16    04/14/16
  59561199    04/15/16    04/15/16
  NaN         04/16/16    04/16/16
  59561198    NaN         04/17/16
  </code></pre>
  
  <p>It should look like below</p>
  
  <pre><code>ID          Date1       Date2
  58844880    04/11/16    NaN
  59743311    04/13/16    NaN
  59598413    NaN         NaN
  59561198    NaN         04/17/16
  </code></pre>
  
  <p>Tried the code
  <code>df = (df['Date1'].isnull() | df['Date1'].isnull())</code></p>