Avoid NPE when creating a relation to an empty document

How to reproduce:

• Click on "Add a new relation" link in relations tab of an asset.
• Selected Predicate as "References".
• Selected Object as Document (Radio button selected) and value not entered.
• Click On "Add" button --> NPE in logs:

   java.lang.NullPointerException
  	at org.nuxeo.ecm.platform.relations.web.listener.ejb.RelationCreationBean.validateObject(RelationCreationBean.java:91)
  	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  	at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
  	at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
  	at java.lang.reflect.Method.invoke(Unknown Source)
  	at org.jboss.el.util.ReflectionUtil.invokeMethod(ReflectionUtil.java:335)
  	at org.jboss.el.util.ReflectionUtil.invokeMethod(ReflectionUtil.java:348)
  	at org.jboss.el.parser.AstPropertySuffix.invoke(AstPropertySuffix.java:58)
  	at org.jboss.el.parser.AstValue.invoke(AstValue.java:96)
  	at org.jboss.el.MethodExpressionImpl.invoke(MethodExpressionImpl.java:276)
  	at com.sun.faces.facelets.el.TagMethodExpression.invoke(TagMethodExpression.java:105)
  	at javax.faces.validator.MethodExpressionValidator.validate(MethodExpressionValidator.java:106)
  	at javax.faces.component.UIInput.validateValue(UIInput.java:1165)
  	at javax.faces.component.UIInput.validate(UIInput.java:983)
  	at javax.faces.component.UIInput.executeValidate(UIInput.java:1249)
  	at javax.faces.component.UIInput.processValidators(UIInput.java:712)
  	at javax.faces.component.UIComponentBase.processValidators(UIComponentBase.java:1258)
  	at javax.faces.component.UIComponentBase.processValidators(UIComponentBase.java:1258)
  	at javax.faces.component.UIForm.processValidators(UIForm.java:253).