add placeholder for empty short description fields

Added a placeholder for fields where short_field_description would be used, but is null. Implemented by adding a default value in FieldDescripton's options, that can be optionally set to false to hide the placeholder, or given any value by the user. Updated docs, translation for english and unit test.

A minor BC occurs, as the placeholder has a default non null value. Requires merge of the matching SonataAdminBundle PR.