Guard against nil dereferenced_target

What does this MR do?

fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/60076

guards against an empty dereferenced_target, which can happen if

1. a tag points to a blob or tree
2. a tag points to a tag...and is more than 10 tag references deep

related: gitaly!1193 (merged)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team