Fix RfC filter for unnamed study groups

app/models/study_group.rb

@@ -16,10 +16,6 @@ class StudyGroup < ApplicationRecord
   end
 
   def to_s
-    if name.blank?
-      "StudyGroup " + id.to_s
-    else
-      name
-    end
+    name.presence || "StudyGroup #{id}"
   end
 end

app/views/request_for_comments/index.html.slim

@@ -10,7 +10,7 @@ h1 = RequestForComment.model_name.human(count: 2)
       = f.select(:solved_not_eq,  [[t('request_for_comments.show_all'), 2], [t('request_for_comments.show_unsolved'), 1], [t('request_for_comments.show_solved'), 0]])
   .form-group
     = f.label(:submission_study_group_id_eq,  t('request_for_comments.index.study_groups.placeholder'), class: 'sr-only')
-    = f.grouped_collection_select(:submission_study_group_id_in, @study_groups_grouping, :second, :first, :id, :name, {},
+    = f.grouped_collection_select(:submission_study_group_id_in, @study_groups_grouping, :second, :first, :id, :to_s, {},
             { class: 'form-control', multiple: true, "data-placeholder": t('request_for_comments.index.study_groups.placeholder') })
 
 .table-responsive