Add check to validate gitlab-version is set for fetch-assets

What does this MR do?

I noted in https://gitlab.com/gitlab-org/build/omnibus-gitlab-mirror/-/jobs/3775879296 that in some error situations we might find ourselves in a position where a nil/empty valid gets passed through.
When the gitlab_version string is empty, we would hit an error registry.gitlab.com/gitlab-org/gitlab/gitlab-assets-ee: is not available yet. Will retry in 30 seconds which would repeat until the pipeline timeout is hit.

We should instead consider failing early when we know we have an invalid tag.

Related issues

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened