Fixed all submissions download.

vpl.class.php

@@ -1130,7 +1130,10 @@ class mod_vpl {
             $nostudents [$user->id] = true;
         }
         $students = array ();
-        $fields = user_picture::fields( 'u' ) . $extrafields;
+        $fields = user_picture::fields( 'u' );
+        if ($extrafields > '') {
+            $fields .= ',' . $extrafields;
+        }
         $all = get_users_by_capability( $this->get_context(), VPL_SUBMIT_CAPABILITY, $fields,
                 'u.lastname ASC', '', '', $group );
         // TODO the following code is too slow.