title: How to convert empty arrays to nulls?
body: <p>I have below dataframe and i need to convert empty arrays to null.</p>
  
  <pre><code>+----+---------+-----------+
  |  id|count(AS)|count(asdr)|
  +----+---------+-----------+
  |1110| [12, 45]|   [50, 55]|     
  |1111|       []|         []|    
  |1112| [45, 46]|   [50, 50]|   
  |1113|       []|         []|
  +----+---------+-----------+
  </code></pre>
  
  <p>i have tried below code which is not working.</p>
  
  <pre><code>df.na.fill("null").show()
  </code></pre>
  
  <p>expected output should be </p>
  
  <pre><code>+----+---------+-----------+
  |  id|count(AS)|count(asdr)|
  +----+---------+-----------+
  |1110| [12, 45]|   [50, 55]|     
  |1111|     NUll|       NUll|    
  |1112| [45, 46]|   [50, 50]|   
  |1113|     NUll|       NUll|
  +----+---------+-----------+
  </code></pre>