fix: Don't throw an exception if no pending licenses were found

It seems plausible to me that this would occasionally happen as part of
the normal flow of things, so instead of alerting on-call just log a
warning.

license_manager/apps/api/tasks.py

@@ -166,6 +166,12 @@ def _batch_notify_or_remind_assigned_emails(
     if len(pending_licenses) > allowed_batch_size:
         raise Exception(f'Found more than {allowed_batch_size} licenses, no email sent to {action_type}')
 
+    if len(pending_licenses) == 0:
+        logger.warning(
+            f'{LICENSE_DEBUG_PREFIX} No pending licenses found for given emails, no email sent to {action_type}.'
+        )
+        return pending_licenses
+
     enterprise_api_client = EnterpriseApiClient()
     enterprise_customer = enterprise_api_client.get_enterprise_customer_data(
         subscription_plan.enterprise_customer_uuid,