Undetached JcrItemModel log when validating an empty sub-field inside the content-block field

The warning logs only display in dev-mode.

Reproduction path:

1. Create a new MyHippoProject
2. Use Essentials to add content-block plugin and create a doc-type using content-block field.
3. In the CMS editor, create a new document of given doc-type in the previous step.
4. Add a new sub-field in the content-block field but leave it empty (please check screenshot).
5. Click 'Save'.
6. The console produces the following warning logs:

   INFO] [talledLocalContainer] 12.10.2015 14:07:19 WARN  http-nio-8080-exec-3 [JcrItemModel.writeObject:352] Undetached JcrItemModel /hippo:namespaces/hippo/mirror/hipposysedit:nodetype/hipposysedit:nodetype
   [INFO] [talledLocalContainer] 12.10.2015 14:07:19 WARN  http-nio-8080-exec-3 [JcrItemModel.writeObject:352] Undetached JcrItemModel /hippo:namespaces/hippo/mirror/hipposysedit:nodetype/hipposysedit:nodetype/tree
   [INFO] [talledLocalContainer] 12.10.2015 14:07:19 WARN  http-nio-8080-exec-3 [DetachMonitor.writeObject:52] Undetached DetachMonitor
   [INFO] [talledLocalContainer] 12.10.2015 14:07:19 WARN  http-nio-8080-exec-3 [JcrItemModel.writeObject:352] Undetached JcrItemModel /hippo:namespaces/system/String/hipposysedit:nodetype/hipposysedit:nodetype