title: Airflow jinja templating: Get param or else execution date (ds_nodash)
body: <p>In my template (XXX.sql) I have:</p>
  
  <pre><code>{{ params.etl_date if params.etl_date is not none else execution_date.strftime('%Y%m%d') }}
  </code></pre>
  
  <p>Then in my BigQueryOperator:</p>
  
  <pre><code>BigQueryOperator(task_id='XXX',
                   bigquery_conn_id=GOOGLE_CLOUD_PLATFORM_CONNECTION_ID,
                   write_disposition='WRITE_TRUNCATE',
                   create_disposition='CREATE_IF_NEEDED',
                   sql='XXX.sql',
                   schema=None,
                   use_legacy_sql=False,
                   destination_dataset_table=XXX,
                   params={
                     'etl_date': '20200605'
                   })
  </code></pre>
  
  <p>it works but with:</p>
  
  <pre><code>BigQueryOperator(task_id='XXX',
                   bigquery_conn_id=GOOGLE_CLOUD_PLATFORM_CONNECTION_ID,
                   write_disposition='WRITE_TRUNCATE',
                   create_disposition='CREATE_IF_NEEDED',
                   sql='XXX.sql',
                   schema=None,
                   use_legacy_sql=False,
                   destination_dataset_table=XXX,
                   # NO PARAMS
                   )
  </code></pre>
  
  <p>I can seem to access and format the execution date? It just returns an empty string. Any advice?</p>