Netlify CMS topics template patch

Why is this change being made?

This MR removes the directory for includes/cms/child_topics. We are using all the same includes and template for child topic pages as parent topic pages.

This MR also adds a if statement so Middleman doesn't fail if body content is missing in the CTA block of the topic template.

Closes https://gitlab.com/gitlab-com/marketing/inbound-marketing/growth/-/issues/1034

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files. (this requirement has been removed pending identification of a new DRI for the handbook)

Closes #1034 (closed)