title: Use Spark when function without otherwise but keep column values
body: <p>I frequently find myself replacing values in columns using</p>
  
  <pre><code>when($"myCol".isNull,myCrazyFunction).otherwise($"myCol")
  </code></pre>
  
  <p>To me the <code>.otherwise($"myCol")</code> feels kind of redundant. </p>
  
  <p>Is there a better way to replace some values under some condition and otherwise just leave everything as it is without using the otherwise?</p>