String className = attributes.getValue(CLASS_ATTRIBUTE);
if (OptionHelper.isEmpty(className)) {
    addError("Missing class name for shutdown hook. Near [" + name + "] line " + getLineNumber(ic));
    inError = true;
    return;
}

<configuration debug="true">
   <!-- in the absence of the class attribute, assume 
   ch.qos.logback.core.hook.DelayingShutdownHook -->
   <shutdownHook/>
  .... 
</configuration>