title: Jaxb unmarshalling does not fail on missing attribute
body: <p>I have a class annotated with jaxb annotations and I want to configure the unmarshaller to fail the unmarshalling if there are missing attributes.</p>
  
  <p>Let's say, I'm having the below class, and I would like to fail the unmarshaling if the EventId attribute is missing from the xml, but instead, it is set to null.</p>
  
  <pre><code>@XmlAccessorType(XmlAccessType.FIELD)
  @XmlRootElement(name = "event")
  public class Event {
  
      @XmlAttribute(name = "EventId", required="true")
      private Integer eventId;
      ...
  }
  </code></pre>
  
  <p>Unmarshalling code:</p>
  
  <pre><code>JAXBContext jaxbContext = JAXBContextFactory.createContext(new Class[]{Events.class}, null);
  StringReader xml = new StringReader(data);
  Unmarshaller unmarshaller = jaxbContext.createUnmarshaller();
  unmarshaller.setEventHandler(new DefaultValidationEventHandler());
  unmarshaller.unmarshal(xml);
  </code></pre>
  
  <p>Am I missing something? Should I configure somehow the unmarshaller to fail if there are missing "required" attributes?</p>
  
  <p>Thanks!</p>