Merge branch 'fix-thumbnail-initializer' into 'master'

Fix Thumbnail Initializer

Avoid confusion between "nil", "empty" or "exists". Now is always "empty" or "exists", no matter what it is in the database.

See merge request !84

app/builders/learning_object_builder.rb

@@ -27,7 +27,7 @@ class LearningObjectBuilder
           id: args['@rid'],
           name: args['name'],
           description: args['description'],
-          thumbnail: args['thumbnail'],
+          thumbnail: (args['thumbnail'] or ''),
           id_dspace: args['id_dspace'],
           type: args['type'],
           bitstreams: args['bitstreams'],