title: undefined getStaticProps causes build failure on page that doesn't exist
body: <p>I'm fetching page data from a cms, so far I have only one page in pages/posts.</p>
  <pre class="lang-js prettyprint-override"><code>
  pages/posts/[slug].js
  
  import { getAllPostsWithSlug, getPostAndMorePosts } from '../../lib/api';
  
  export default function Post({ post }) {
    const router = useRouter();
    const { slug } = router.query;
    return (
      &lt;div&gt;
        &lt;p&gt;
          title: {typeof post == 'undefined' ? 'no post' : post.title}
        &lt;/p&gt;
      &lt;/div&gt;
    );
  }
  
  export async function getStaticProps({ params, preview = null }) {
    const data = await getPostAndMorePosts(params.slug, preview);
    const content = await markdownToHtml(data?.posts[0]?.content || '');
  
    return {
      props: {
        preview,
        post: {
          ...data?.posts[0],
          content,
        },
        morePosts: data?.morePosts,
      },
    };
  }
  
  export async function getStaticPaths() {
    const allPosts = await getAllPostsWithSlug();
    return {
      paths: allPosts?.map((post) =&gt; `/posts/${post.slug}`) || [],
      fallback: true,
    };
  }
  
  </code></pre>
  <p>That will correctly display post.title, but if I access the property directly with</p>
  <pre><code>&lt;p&gt;title: {post.title}&lt;/p&gt;
  </code></pre>
  <p>I get the build error:
  <a href="https://i.stack.imgur.com/vFvrv.png" rel="nofollow noreferrer">post undefined</a></p>
  <p>Is next trying to build a page out of the template with no data? When the build succeeds I only have one route in /posts.</p>