title: CSV data source does not support null data type in pyspark
body: <p>I am trying to save dataframe into file using the pyspark method but I am getting above error. IThe same code I am trying in python 2.7, there it is working but python 3.6 it is not working. Could anyone help me what I am doing wrong?
  I am adding the schema details of the table.</p>
  <p>I am trying below code:</p>
  <pre><code>df.repartition(1).write.format('com.databricks.spark.csv').mode('overwrite').save(output_path,quote='',escape='\&quot;', sep='|',header='True',nullValue=None)
  </code></pre>
  <p>PrintSchema():</p>
  <pre><code>|-- fact_id: integer (nullable = true)
   |-- Active_Flag: null (nullable = true)
   |-- Project_End_Date: string (nullable = true)
   |-- Project_Effective_Date: string (nullable = true)
   |-- Notification to Implement Issued Clarification: string (nullable = true)
   |-- Notification to Implement Issued: string (nullable = true)
   |-- pERC Reconsideration Meeting Clarification: string (nullable = true)
   |-- pERC Reconsideration Meeting: string (nullable = true)
   |-- Feedback Deadline Clarification: string (nullable = true)
   |-- Feedback Deadline: string (nullable = true)
   |-- pERC Meeting Clarification: string (nullable = true)
   |-- pERC Meeting: string (nullable = true)
   |-- Check-point meeting Clarification: string (nullable = true)
   |-- Check-point meeting: string (nullable = true)
   |-- Patient Advocacy Group Input Deadline Clarification: string (nullable = true)
   |-- Patient Advocacy Group Input Deadline: string (nullable = true)
   |-- NOC Date: string (nullable = true)
   |-- Pre NOC Submission: string (nullable = true)
   |-- Status Clarification: string (nullable = true)
   |-- Review Status: string (nullable = true)
   |-- Prioritization: string (nullable = true)
   |-- Tumour Type: string (nullable = true)
   |-- Submitter: string (nullable = true)
   |-- Sponsor: string (nullable = true)
   |-- Funding Request: string (nullable = true)
   |-- CADTH review report(s) posted: string (nullable = true)
   |-- Deadline for sponsor to submit redaction requests on draft CADTH review report(s): string (nullable = true)
   |-- Reconsideration Requested Clarification: string (nullable = true)
   |-- Reconsideration Requested: string (nullable = true)
   |-- Final Recommendation Issued Clarification: string (nullable = true)
   |-- Final recommendation posted: string (nullable = true)
   |-- Final Recommendation Sent to Drug Plans and Manufacturer Date: string (nullable = true)
   |-- Reconsideration Meeting Date Clarification: string (nullable = true)
   |-- Reconsideration Meeting Date: string (nullable = true)
   |-- Embargo period ended clarification: null (nullable = true)
   |-- Embargo period ended: null (nullable = true)
   |-- Embargo Period Date Clarification: string (nullable = true)
   |-- Embargo Period Date: string (nullable = true)
   |-- Final recommendation issued to sponsor and drug plans Clarification: string (nullable = true)
   |-- Final recommendation issued to sponsor and drug plans: string (nullable = true)
   |-- CDEC Meeting Date Clarification: string (nullable = true)
   |-- CDEC Meeting Date: string (nullable = true)
   |-- Redaction Response from Manufacturer on Report Received by CADTH Date Clarification: string (nullable = true)
   |-- Redaction Response from Manufacturer on Report Received by CADTH Date: string (nullable = true)
   |-- Comments from Manufacturers on Reviewer's Report Date Clarification: null (nullable = true)
   |-- Comments from Manufacturers on Reviewer's Report Date: null (nullable = true)
   |-- Deadline for sponsors comments Clarification: string (nullable = true)
   |-- Deadline for sponsors comments: string (nullable = true)
   |-- CADTH Reviewers Report sent to Manufacturer Date Clarification: string (nullable = true)
   |-- CADTH Reviewers Report sent to Manufacturer Date: string (nullable = true)
   |-- Patient Group Input Summary Comment Received Date Clarification: string (nullable = true)
   |-- Patient Group Input Summary Comment Received Date: string (nullable = true)
   |-- Patient input summary sent for review to patient input groups clarification: string (nullable = true)
   |-- Patient input summary sent for review to patient input groups: string (nullable = true)
   |-- Patient Group Input Submission Received Date Clarification: string (nullable = true)
   |-- Patient Group Input Submission Received Date: string (nullable = true)
   |-- Call for Patient Input Date Clarification: string (nullable = true)
   |-- Call for Patient Input Date: string (nullable = true)
   |-- Submission Deemed Complete Actual Date Clarification: string (nullable = true)
   |-- Submission Deemed Complete Actual Date: string (nullable = true)
   |-- Submission Deemed Complete Target Date Clarification: string (nullable = true)
   |-- Submission Deemed Complete Target Date: string (nullable = true)
   |-- Patient group input closed clarification: string (nullable = true)
   |-- Patient group input closed: string (nullable = true)
   |-- Fee Schedule: string (nullable = true)
   |-- Recommendation Type: string (nullable = true)
   |-- Initial Recommendation Issued Clarification: string (nullable = true)
   |-- Recommendation Date: string (nullable = true)
   |-- Companion Diagnostics: string (nullable = true)
   |-- Submission Type: string (nullable = true)
   |-- Review initiated: string (nullable = true)
   |-- Submission Date Clarification: string (nullable = true)
   |-- Submission accepted: string (nullable = true)
   |-- Submission received: string (nullable = true)
   |-- Manufacture_id: string (nullable = true)
   |-- Biosimilar: string (nullable = true)
   |-- Manufacturer Requested Reimbursement Criteria: string (nullable = true)
   |-- Thrc_area_id: string (nullable = true)
   |-- Generic_id: string (nullable = true)
   |-- BRAND_ID: string (nullable = true)
   |-- indication_id: string (nullable = true)
   |-- Strength: string (nullable = true)
   |-- Project Number: string (nullable = true)
   |-- Modified Time: string (nullable = true)
   |-- Published Time: string (nullable = true)
   |-- Updated Time: string (nullable = true)
   |-- Program Name: string (nullable = true)
  </code></pre>