Prevent build failures when featured pages are not included

What does this MR do and why?

Fixes a regression introduced in !4844 (merged).

In the upstream review app pipeline, we do partial builds (without all source content). This MR allows the site to continue building even if some featured pages do not exist, like on these pipelines.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.
2. Remove gitlab-runner from your build and compile the site.
3. Even though there are gitlab-runner pages featured on the homepage, the build will not crash when trying to validate them.

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.