[BACKPORT] Facet navigation tree showing draft documents

1. create a project with the archetype version 1.07.04, add the latest hst version to the project: <hippo.hst.version>2.26.06</hippo.hst.version> (maybe not necessary)
2. build and run it, in the cms open a news document and click "edit" (so you have also a draft version of it).
3. on http://localhost:8080/cms/console/?path=/content/documents/myhippoproject/news import the attached newsfacet.xml
4. set the hippo:docbase of the new newsfacet node to the uuid of the /content/documents/myhippoproject/news node
5. write changes to repository: instead of 3 documents in the resultset I have 4: also the draft version (see attached screenshot)

The bug seems to be in the JDO mapping, with an empty array being persisted as null.