A bunch of tiny little fixes for issues I found while setting up docusaurus (#312)

* Do not show users related UI if no users have been specified

* Fixed typo in Docusaurus homepage link

* Allow projects without icons

Not every project has an icon, so avoid showing a broken image in that case.

* Do not show project icon in the footer if there's no footer icon

examples/basics/pages/en/index.js

@@ -167,6 +167,9 @@ const Description = props => (
 );
 
 const Showcase = props => {
+  if ((siteConfig.users||[]).length === 0) {
+    return null;
+  }
   const showcase = siteConfig.users
     .filter(user => {
       return user.pinned;