fixing download bug when there was no attachment

Signed-off-by: man13 <man13@inf.ufpr.br>

app/helpers/learning_objects_helper.rb

@@ -59,6 +59,7 @@ module LearningObjectsHelper
   end
 
   def display_download_button(learning_object)
+    return nil if learning_object.default_attachment.blank?
     return external_website_btn(learning_object) if learning_object.has_url_reference?
     download_btn learning_object
   end