Added empty before_script block in container_scanning

This change is done to fix container scanning failure in wellbore worker pipeline - https://community.opengroup.org/osdu/platform/domain-data-mgmt-services/wellbore/wellbore-domain-services-worker/-/jobs/3006657

The failure is due to the presence of default block in wellbore worker pipeline which is running pip3 commands as a before_script step on the container_scanning job. Adding an empty [] block in container_scanning prevents the defaault block from being run.

This change doesn't affect the functionality for other services where default is not present.