Download metadata

Do not offer the download option if no metadata available 
see https://trac.openmicroscopy.org.uk/ome/ticket/11710

To test:
 * log in as user-1
 * Expand ```MP_proj > MP_ds``` 
 * Select the image ending with ```dv.offsets``` (id 14142)
 * Go to the Acquisition tab 
 * Expand the Original metadata pane
 * The download button should be greyed out.