Download crash dev 4 4

Manually fix issue already in develop cf. gh-1889. A previous PR has not been rebased (zip)

Fix NPE when downloading the archived file see http://trac.openmicroscopy.org.uk/ome/ticket/11810

* Log in as (see sheer)
* Select an image with archived flag (see screenshot attached to ticket)
* Click on the "Download archived"
* Message indicating that no archived available should be displayed.