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
Did you mean?  concern
  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
Did you mean?  concern

ActionView::Template::Error: undefined method `content' for nil:NilClass
Did you mean?  concern

cc: @adamniedzielski

added ~874211 Next Patch Release regression labels

changed milestone to %9.1

mentioned in issue #30864 (closed)

assigned to @adamniedzielski

I identified a valid case where node.children is empty:

[](https://gitlab.com/gitlab-org/gitlab-ce/issues/30874)

That is: an empty link to issue (or merge request).

However, we don't have this case in our CHANGELOG.md. The line that is failing is:

## 8.8.9

That's because it's transformed into following HTML (roughly, because that's from my development machine):

<h2 dir="auto">
<a href="/gitlab-org/gitlab-ce/issues/899" data-original="" data-link="true" data-project="2" data-issue="2316" data-reference-type="issue" title="Excepturi iure dicta officiis ut fugit quaerat." class="gfm gfm-issue has-tooltip"></a>899</h2>

This seems like a bug in our reference parser, possibly a long standing one (because it doesn't change the visible output) and separate from this issue.

mentioned in commit 551d0a3c

mentioned in merge request !10683 (merged)

mentioned in issue #30930 (moved)

closed via merge request !10683 (merged)

mentioned in commit 7d7dfee4

mentioned in issue #30964 (closed)

mentioned in issue #30972 (closed)

mentioned in commit 06afe8b4

mentioned in issue #30982 (closed)

mentioned in issue #31032 (closed)

mentioned in issue #31013 (closed)