If the example is cloned and ran with no further setup (no environment variables set up), it will error out because `extractPostEntries` will return `undefined` which can not be serialized by `getStaticProps `.
This PR basically ensures that we return an empty array instead of the `undefined` for a collection of posts, and a `null` instead of the `undefined` for a single post.
[ch2502]
- Added new import flow to every CMS example, using the deploy button, and removed `vercel.json`
- Replaced `dotenv` with the new env support in outdated examples
