title: Is it possible to ignore null values when using LEAD window function in Spark?
body: <p>My dataframe like this</p>
  <pre class="lang-none prettyprint-override"><code>id  value  date    
  1   100    2017 
  1   null   2016 
  1   20     2015 
  1   100    2014
  </code></pre>
  <p>I would like to get most recent previous value but ignoring null</p>
  <pre class="lang-none prettyprint-override"><code>id  value  date   recent value
  1   100    2017    20
  1   null   2016    20
  1   20     2015   100
  1   100    2014   null
  </code></pre>
  <p>Is there any way to ignore null values while using <code>lead</code> window function?</p>