Error 500 rendering an issue due to Markdown redaction

Created by @stanhu.

https://sentry.gitlap.com/gitlab/gitlabcom/issues/27662/

Sample failing URL: https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/1421

NoMethodError: undefined method `feature_available?' for nil:NilClass
  app/models/issue.rb:224:in `visible_to_user?'
    return false unless project.feature_available?(:issues, user)
  app/models/ability.rb:43:in `block in issues_readable_by_user'
    issues.select { |issue| issue.visible_to_user?(user) }
  app/models/ability.rb:43:in `select'
    issues.select { |issue| issue.visible_to_user?(user) }
  app/models/ability.rb:43:in `issues_readable_by_user'
    issues.select { |issue| issue.visible_to_user?(user) }
  lib/banzai/reference_parser/issue_parser.rb:13:in `nodes_visible_to_user'
    issues_readable_by_user(issues.values, user).to_set
...
(133 additional frame(s) were not displayed)

NoMethodError: undefined method `feature_available?' for nil:NilClass