Don't add "Previous version" when mapped_page is to a versionless book

[colleenmcginnis]

Describe the bug

For mapped_pages that link to versionless AsciiDoc books (paths set to undefined in legacy-url-mappings.yml), we should not add a "Previous versions" link because it just redirects back to the v3 page.

Example: https://www.elastic.co/docs/deploy-manage/users-roles

Expected behavior

The "Previous version" should be grayed out.

Steps to reproduce

1. Go to https://www.elastic.co/docs/deploy-manage/users-roles
2. Open the version dropdown.
3. Click "Previous version".
4. You will be redirected to the same page you started on.

Tooling

• docs-builder
• migration tooling
• I'm not sure

[reakaleek]

@cotti can we just remove every entry with undefined as value?

Would that have the desired outcome?

[cotti]

@cotti can we just remove every entry with undefined as value?

Would that have the desired outcome?

Nope, we need a minor adjustment on the mapping logic. If we don't find a viable mapping, we are still yielding a LegacyPageMapping with empty values there rather than null. Skimming through its usage here, seems safe to just allow it to return null. The method's signature already predicts it anyway, and it's the return in case we don't have a list of mapped pages in the first place.