Forward port (REPO-528): Altering order of an empty directory causes nullpointer exception

See REPO-528
[HippoCMS-svn 36765]

Changing the order of an empty directory causes a nullpointer exception which is visible in the hippo-cms.log.

Reproduction path:

• Login to the CMS
• Select the documents overview
• Select a directory (ie. News)
• Create a new subdirectory
• Select the new subdirectory
• Select option 'Order' from popup menu
• Select one of the buttons (Up or Down) to alter the order

--> nullpointer exception appears in hippo-cms.log :

27.06.2012 12:00:28 ERROR [org.apache.wicket.RequestCycle.logRuntimeException():1531]
java.lang.NullPointerException