only trigger DVLA tasks if there is data to send

app/celery/scheduled_tasks.py

@@ -316,6 +316,7 @@ def populate_monthly_billing():
 @statsd(namespace="tasks")
 def run_letter_jobs():
     job_ids = dao_get_letter_job_ids_by_status(JOB_STATUS_READY_TO_SEND)
+    if job_ids:
         notify_celery.send_task(
             name=TaskNames.DVLA_JOBS,
             args=(job_ids,),
@@ -331,6 +332,7 @@ def run_letter_notifications():
 
     notifications = dao_set_created_live_letter_api_notifications_to_pending()
 
+    if notifications:
         file_contents = create_dvla_file_contents_for_notifications(notifications)
 
         file_location = '{}-dvla-notifications.txt'.format(current_time)