title: Airflow: SQL to GCS error with NULL datetime fields
body: <p>I'm using the <strong>MsSqlToGoogleCloudStorageOperator</strong> operator to extract data from my MSSQL server DB, and I noticed that if I query tables that have <strong>datetime</strong> fields using a <strong>SELECT * FROM [table_name</strong>] my task will fail with a</p>
  <pre><code>ERROR - Object of type datetime is not JSON serializable
  </code></pre>
  <p>The only round this has been to specify the fields I want and even in that case if there's a datetime field that's <strong>NULL</strong> I get the same error. Any clue on how to fix this?</p>