Fix version format when content is `null`

There is an edge case that formatVersion method does not take into account.
In some documents major and minor versions can be `null` (see support ticket).

We should had that exception to the method to not present anything in these situations.

Reproduction steps

• create a new document type on a studio project with facet folderish.
• this will include the UID schema (cannot remove in studio afaik)
• on web ui, create some documents of this type.
• select them and copy to any place.
• select the "version" on the data table field list
  =!> you get `null.null`

Video
https://jira.nuxeo.com/secure/attachment/82089/82089_2019-06-14_11-03-08.mp4