feature: cpd-945 return a landing page if default not set

[itsmostafa]

Handle a potential bug in case default landing pages don't exist. Return the first landing page so that Subsriptions don't break.

🗣 Description

💭 Motivation and context

🧪 Testing

tested locally

✅ Pre-approval checklist

• [x] This PR has an informative and human-readable title.
• [x] Changes are limited to a single goal - eschew scope creep!
• [x] All future TODOs are captured in issues, which are referenced in code comments.
• [x] All relevant type-of-change labels have been added.
• [x] I have read the CONTRIBUTING document.
• [x] These code changes follow cisagov code standards.
• [x] All relevant repo and/or project documentation has been updated to reflect the changes in this PR.
• [x] Tests have been added and/or modified to cover the changes in this PR.
• [x] All new and existing tests pass.