Empty Tag returned by 'v2/<REPO>/tags/list' when S3 returns 'Contents' as well as 'CommonPrefixes' on ListObjects

Typically S3 returns just CommonPrefixes when querying a directory. However, in some cases it may return the _tags directory passed as prefix as Content as well. The S3 backend will return both files and directories, so we strip the full tags directory path and return an empty string to the caller.

This in turn will cause the UI to error out when attempting to view tags because the tag name is empty:

ActionController::UrlGenerationError (No route matches {:action=>"destroy", :controller=>"projects/registry/tags", :id=>"", :namespace_id=>#<Group id:740 @dcube_core>, :project_id=>#<Project id:5296 dcube_core/python_dienst_template>, :repository_id=>#<ContainerRepository id: 1, project_id: 5296, name: "", created_at: "2019-02-27 09:06:06", updated_at: "2019-02-27 09:06:06">} missing required keys: [:id]):

Example output from S3:

{
  "Contents": [
    {
      "Key": "docker/registry/v2/repositories/group/project/production/_manifests/tags/",
      "LastModified": "2021-03-03T09:04:14+00:00",
      "ETag": "\"123abc\"",
      "Size": 0,
      "StorageClass": "STANDARD",
      "Owner": {
        "DisplayName": "abc123",
        "ID": "12345"
      }
    }
  ],
  "CommonPrefixes": [
    {
      "Prefix": "docker/registry/v2/repositories/group/project/production/_manifests/tags/tag1/"
    },
    {
      "Prefix": "docker/registry/v2/repositories/group/project/production/_manifests/tags/tag2/"
    },
    {
      "Prefix": "docker/registry/v2/repositories/group/project/production/_manifests/tags/tag3/"
    }
  ]
}

/cc @hswimelar