Only set 'url' property if actually submitted by the client (#2876)

plugins/calendar/calendar.php

@@ -1654,9 +1654,11 @@ class calendar extends rcube_plugin
     }
 
     // mapping url => vurl because of the fullcalendar client script
+    if (array_key_exists('vurl', $event)) {
       $event['url'] = $event['vurl'];
       unset($event['vurl']);
     }
+  }
 
   /**
    * Releases some resources after successful event save