title: UnsupportedOperationException when trying to save dataframe to redshift
body: <p>I am working with pyspark and saving dataframe to redshift. I am getting the below error when trying to save it to</p>
  
  <pre><code>": java.lang.UnsupportedOperationException: Unexpected type NullType.
          at com.databricks.spark.avro.SchemaConverters$.com$databricks$spark$avro$SchemaConverters$$convertFieldTypeToAvro(SchemaConverters.scala:283)
  "
  </code></pre>
  
  <p>When I look athte source code for ScehmaCovnerters : 
  <a href="https://github.com/databricks/spark-avro/blob/master/src/main/scala/com/databricks/spark/avro/SchemaConverters.scala" rel="nofollow">https://github.com/databricks/spark-avro/blob/master/src/main/scala/com/databricks/spark/avro/SchemaConverters.scala</a></p>
  
  <p>I can see no support for NullType. I have columns in dataframe which are null.</p>
  
  <p>What is the solution to this?</p>