title: Remove Nulls in specific Rows in Dataframe and combine rows
body: <p>Need to do the below activity in Spark Dataframes using Scala.
  Have tried doing some basic filters isNotNull conditions and others. But no luck.</p>
  
  <p>Input</p>
  
  <pre class="lang-none prettyprint-override"><code>+----------+----------+----------+
  |     Amber|     Green|       Red|
  +----------+----------+----------+
  |      null|      null|[AE,AA,CV]|
  |      null|[AH,EE,CC]|      null|
  |[DD,DE,QQ]|      null|      null|
  +----------+----------+----------+
  </code></pre>
  
  <p>Output</p>
  
  <pre class="lang-none prettyprint-override"><code>+----------+----------+----------+
  |     Amber|     Green|       Red|
  +----------+----------+----------+
  |[DD,DE,QQ]|[AH,EE,CC]|[AE,AA,CV]|
  +----------+----------+----------+
  </code></pre>