Container Registry image tags can be invalid which prevents deletion
### Summary
Investigating issues with image tag deletion and there are tags in the system which do not have a digest. Without the digest a deletion cannot be carried out
### Steps to reproduce
Unsure of steps to reproduce but we have projects that on GitLab.com with invalid tags
### Example Project
https://gitlab.com/vais/tts-worker-hts/container_registry
### What is the current *bug* behavior?
Image tag deletion does nothing
### What is the expected *correct* behavior?
Image tag deletion should remove the tag
### Relevant logs and/or screenshots
(Paste any relevant logs - please use code blocks (```) to format console output,
logs, and code as it's very hard to read otherwise.)
### Output of checks
GitLab.com v10.1.3
### Possible fixes
(If you can, link to the line of code that might be responsible for the problem)
issue