Fix empty version label after copy + versioning

Step to reproduce:

• select a File document in a content view and push the Copy button
• push the Paste button
• a new document is created in the same folder: get its id (cd551cce-5322-4368-b045-e13086f250bb in my example)
• edit the new document and increment the minor version
• run the following SQL query against your database

  select * from versions where versionableid='cd551cce-5322-4368-b045-e13086f250bb';
                    id                  |            versionableid             |         created         | label | description | islatest | islatestmajor 
  --------------------------------------+--------------------------------------+-------------------------+-------+-------------+----------+---------------
   ba10d4af-9045-48bc-be46-0c293b84b408 | cd551cce-5322-4368-b045-e13086f250bb | 2016-05-26 14:38:00.815 |       |             | t        | f
  

  => the label value is empty

This value is returned when calling session.getLastVersion(docRef).getLabel().