Fix appending state to issuable references

What does this MR do?

Ignore nodes with no children. Append directly to the node instead of the last child of the node to avoid inheriting formatting from the last child.

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/30874

mentioned in issue #30864 (closed)

approved this merge request

merged

mentioned in commit 7d7dfee4

I should have picked this for RC2, but too late :x

added regression label

mentioned in commit 06afe8b4

Picked into 9-1-stable, will go into 9.1 RC3

mentioned in issue #31032 (closed)

mentioned in merge request !10782 (merged)