title: Create new Dataframe with empty/null field values
body: <p>I am creating a new Dataframe from an existing dataframe, but need to add new column ("field1" in below code) in this new DF. How do I do so? Working sample code example will be appreciated.</p>
  
  <pre><code>val edwDf = omniDataFrame 
    .withColumn("field1", callUDF((value: String) =&gt; None)) 
    .withColumn("field2",
      callUdf("devicetypeUDF", (omniDataFrame.col("some_field_in_old_df")))) 
  
  edwDf
    .select("field1", "field2")
    .save("odsoutdatafldr", "com.databricks.spark.csv"); 
  </code></pre>