Indicate that `single_discussion` is optional

app/helpers/issues_helper.rb

@@ -134,7 +134,7 @@ module IssuesHelper
     options_from_collection_for_select(options, 'name', 'title', params[:due_date])
   end
 
-  def link_to_discussions_to_resolve(merge_request, single_discussion)
+  def link_to_discussions_to_resolve(merge_request, single_discussion = nil)
     link_text = merge_request.to_reference
     link_text += " (discussion #{single_discussion.first_note.id})" if single_discussion