Backport of 'Fix 500 on multi-arch tags on the legacy registry path' into 18.11

What does this MR do and why?

Backports Fix 500 on multi-arch tags on the legacy regist... (!240746 - merged) • Hayley Swimelar • 19.2 into 18.11.

On the legacy (non-metadata-database) registry path, viewing tag details for a multi-arch tag (Docker manifest list or OCI image index) returns a 500. ContainerRegistry::Tag#layers calls .map on a nil layer list, because a fat manifest carries no image layers. This MR guards #layers, #config_blob, and #total_size against non-image manifests.

The regression comes from the accepted-manifest-type widening in Geo: fix container repository sync for OCI imag... (!236905 - merged) • Douglas Barbosa Alexandre • 19.1, which let the per-tag manifest fetch receive fat manifests. The widening is present on all maintained stable branches (18.10, 18.11, 19.0, 19.1). Bug: Container registry: multi-arch tags 500 on the ... (#603051 - closed) • Hayley Swimelar • 19.2.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
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• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

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