Handle case when author does not exist

According to the Edge type in allMarkdownRemarkData, the author in
frontmatter could be undefined.

related to #24

src/pages/blog/all.html.js

@@ -76,6 +76,7 @@ const AllBlogPosts = ({data}: Props) => (
                 </Link>
               </h2>
               <MetaTitle>{node.fields.date}</MetaTitle>
+              {node.frontmatter.author ? (
                 <div
                   css={{
                     color: colors.subtle,
@@ -88,6 +89,9 @@ const AllBlogPosts = ({data}: Props) => (
                     </span>
                   ))}
                 </div>
+              ) : (
+                <div />
+              )}
             </li>
           ))}
         </ul>