Fix one more issue for username "0".

Author: epriestley

src/applications/auth/oauth/provider/PhabricatorOAuthProviderGitHub.php

@@ -112,7 +112,7 @@ public function retrieveUserProfileImage() {
 
   public function retrieveUserAccountURI() {
     $username = $this->retrieveUserAccountName();
-    if ($username) {
+    if (strlen($username)) {
       return 'https://github.com/'.$username;
     }
     return null;