Fix bug when exposing non existing artifacts

What does this MR do?

Related issue: #121990 (closed)

This MR fixes a bug that occurs when expose_as is used and no artifacts are produced by the job.

Errors were reported in Sentry: https://sentry.gitlab.net/gitlab/gitlabcom/issues/1125946/

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content