title: How to create DataFrame with nulls using toDF?
body: <p>How do you create a dataframe containing nulls from a sequence using .toDF ?</p>
  
  <p>This works:</p>
  
  <pre><code>val df = Seq((1,"a"),(2,"b")).toDF("number","letter")
  </code></pre>
  
  <p>but I'd like to do something along the lines of:</p>
  
  <pre><code>val df = Seq((1, NULL),(2,"b")).toDF("number","letter")
  </code></pre>