[Examples] Prevent getStaticProps from returning "undefined" (#19012)

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.

examples/cms-contentful/pages/index.js

@@ -36,7 +36,7 @@ export default function Index({ preview, allPosts }) {
 }
 
 export async function getStaticProps({ preview = false }) {
-  const allPosts = await getAllPostsForHome(preview)
+  const allPosts = (await getAllPostsForHome(preview)) ?? []
   return {
     props: { preview, allPosts },
   }