title: I need to set column B in a dataframe to null if column A is NULL
body: <p>Given the following dataframe:</p>
  
  <pre><code>A    B   C
  NULL 1   2
  3    4   3
  5    3   1
  NULL 2   6
  </code></pre>
  
  <p>I'd like to replace the value in B with null if the A is null:</p>
  
  <pre><code>A     B    C
  NULL  NULL 2
  3     4    3
  5     3    1
  NULL  NULL 6
  </code></pre>
  
  <p>Any ideas?</p>