title: Filter NULL value in dataframe column of spark scala
body: <p>I have a dataframe(df) with following:</p>
  
  <pre><code>+---------+--------------------+
  |  col1|        col2           |
  +---------+--------------------+
  |colvalue1|                NULL|   
  |colvalue2|col2value...        |
  +------------+-----------------+
  </code></pre>
  
  <p>I am trying to filter rows based on the col2 as follows</p>
  
  <pre><code>df.filter(($"col2".isNotNULL) || ($"col2" !== "NULL")  || ($"col2" !== "null")  || ($"col2".trim !== "NULL"))
  </code></pre>
  
  <p>But the row which has NULL is not filtering. This column show <code>nullable=true</code>.</p>
  
  <p>Can anyone let me know what mistake I am doing? I am using Spark 1.6.</p>