title: How to na.fill an empty array in Apache Spark?
body: <p>I have the schema like this: </p>
  
  <pre><code>root
   |-- id: string (nullable = true)
   |-- MATCH_timestamp: array (nullable = true)
   |    |-- element: long (containsNull = true)
   |-- MATCH_n: integer (nullable = true)
   |-- PAYMENT_INAPP_timestamp: array (nullable = true)
   |    |-- element: long (containsNull = true)
   |-- PAYMENT_INAPP_cash: array (nullable = true)
   |    |-- element: string (containsNull = true)
   |-- PAYMENT_INAPP_coin: array (nullable = true)
   |    |-- element: string (containsNull = true)
   |-- PAYMENT_INAPP_count: integer (nullable = true)
  </code></pre>
  
  <p>However, PAYMENT_INAPP_ stuff may be null, as user might not pay yet. As you see in schema, PAYMENT_INAPP_timestamp, PAYMENT_INAPP_cash, PAYMENT_INAPP_coin is an array. I would like to replace null value with an empty array. </p>
  
  <p>Tried, but not working: </p>
  
  <pre><code>myDf.na.fill(Array.empty[Int], Seq("PAYMENT_INAPP_timestamp", "PAYMENT_INAPP_cash", "PAYMENT_INAPP_coin"))
  
  myDf.na.fill(Array.empty[Long], Seq("PAYMENT_INAPP_timestamp", "PAYMENT_INAPP_cash", "PAYMENT_INAPP_coin"))
  
  myDf.na.fill(Array.empty[String], Seq("PAYMENT_INAPP_timestamp", "PAYMENT_INAPP_cash", "PAYMENT_INAPP_coin"))
  </code></pre>
  
  <p>or</p>
  
  <pre><code>myDf.na.fill(lit(Array.empty[Int]), Seq("PAYMENT_INAPP_timestamp", "PAYMENT_INAPP_cash", "PAYMENT_INAPP_coin"))
  
  myDf.na.fill(lit(Array.empty[String]), Seq("PAYMENT_INAPP_timestamp", "PAYMENT_INAPP_cash", "PAYMENT_INAPP_coin"))
  
  myDf.na.fill(lit(Array.empty[Long]), Seq("PAYMENT_INAPP_timestamp", "PAYMENT_INAPP_cash", "PAYMENT_INAPP_coin"))
  </code></pre>