title: Spark CSV read/write for empty field
body: <p>I want to write by Dataframe's empty field as empty but it always writes as <code>NULL</code>. I want to write NULLS as <code>?</code> and empty as empty/blank. Same while reading from a csv. </p>
  
  <pre><code>val df = sqlContext.createDataFrame(Seq(
      (0, "a"),
      (1, "b"),
      (2, "c"),
      (3, ""),
      (4, null)
  ))    
  
  scala&gt; df.show
  
  |  0|   a|
  |  1|   b|
  |  2|   c|
  |  3|    |
  |  4|null|   
  +---+----+
  
  df.write.mode(SaveMode.Overwrite).format("com.databricks.spark.csv").option("nullValue","?").save("/xxxxx/test_out")
  
  written output :
  
  0,a
  1,b
  2,c
  3,?
  4,?
  .option("treatEmptyValuesAsNulls" , "false")
  </code></pre>
  
  <p>This option does not work.</p>
  
  <p>I need the empty to write as empty </p>
  
  <pre><code>0,a
  1,b
  2,c
  3,
  4,?
  </code></pre>