500 error on CHANGELOG.md for GitLab CE

https://sentry.gitlap.com/gitlab/gitlabcom/issues/26627/

See https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG.md

NoMethodError: undefined method `content' for nil:NilClass
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  lib/banzai/filter/issuable_state_filter.rb:17:in `block in call'
    node.children.last.content += " [#{issuable.state}]"
  lib/banzai/filter/issuable_state_filter.rb:15:in `each'
    issuables.each do |node, issuable|
  lib/banzai/filter/issuable_state_filter.rb:15:in `call'
    issuables.each do |node, issuable|
  lib/gitlab/metrics/instrumentation.rb:156:in `block in call'
    trans.method_call_for(#{label.to_sym.inspect}).measure { super }
  lib/gitlab/metrics/method_call.rb:23:in `measure'
    retval = yield
...
(182 additional frame(s) were not displayed)

ActionView::Template::Error: undefined method `content' for nil:NilClass
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ActionView::Template::Error: undefined method `content' for nil:NilClass
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cc: @adamniedzielski