XML Backup fails if user attributes are empty or NULL

java.lang.IllegalArgumentException: Adding text to an XML document must not be null
	at org.dom4j.DocumentFactory.createText(DocumentFactory.java:142)
	at org.dom4j.tree.AbstractElement.addText(AbstractElement.java:1286)
	at com.atlassian.crowd.migration.UserMapper.addUserAttributesToXml(UserMapper.java:120)
	at com.atlassian.crowd.migration.UserMapper.addUserToXml(UserMapper.java:95)
	at com.atlassian.crowd.migration.UserMapper.exportXml(UserMapper.java:74)
	at com.atlassian.crowd.migration.XmlMigrator.exportXml(XmlMigrator.java:35)
	at com.atlassian.crowd.migration.XmlMigrationManagerImpl.exportXml(XmlMigrationManagerImpl.java:97)
	at com.atlassian.crowd.console.action.admin.Backup.doExport(Backup.java:103)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:592)
	at com.opensymphony.xwork.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:358)
	at com.opensymphony.xwork.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:218)