Fix nil learning object error on check_links task

lib/tasks/attachment_maintaining_service.rake

@@ -53,7 +53,7 @@ require 'fileutils'
             if att_url.blank?
               p "-----------------------------------------------------"
               p "Retrieve URL nil"
-              p "LearningObject id: #{att.learning_object.id}"
+              p "LearningObject id: #{att.learning_object.id}" if !att.learning_object.nil?
               p "Attachment id: #{att.id}, id_dspace: #{att.id_dspace}"
             else
               begin
@@ -61,7 +61,7 @@ require 'fileutils'
               rescue URI::InvalidURIError              
                 p "-----------------------------------------------------"
                 p "Invalid URI: #{att_url}"
-                p "LearningObject id: #{att.learning_object.id}"
+                p "LearningObject id: #{att.learning_object.id}" if !att.learning_object.nil?
                 p "Attachment id: #{att.id}, id_dspace: #{att.id_dspace}"
                 next
               end
@@ -71,7 +71,7 @@ require 'fileutils'
                 p "-----------------------------------------------------"
                 p "Response != 200: #{response.inspect}"
                 p "Link: #{att_url}"
-                p "LearningObject id: #{att.learning_object.id}"
+                p "LearningObject id: #{att.learning_object.id}" if !att.learning_object.nil?
                 p "Attachment id: #{att.id}, id_dspace: #{att.id_dspace}"
               end
             end