[30926] Display info instead of error when toc is empty

https://community.openproject.com/wp/30926

config/locales/en.yml

@@ -1628,6 +1628,7 @@ en:
   label_wiki_content_added: "Wiki page added"
   label_wiki_content_updated: "Wiki page updated"
   label_wiki_toc: "Table of Contents"
+  label_wiki_toc_empty: "Table of Contents is empty as no headings are present."
   label_wiki_dont_show_menu_item: "Do not show this wikipage in project navigation"
   label_wiki_edit: "Wiki edit"
   label_wiki_edit_plural: "Wiki edits"

lib/open_project/text_formatting/filters/table_of_contents_filter.rb

@@ -117,9 +117,13 @@ module OpenProject::TextFormatting
       end
 
       def process!
-        return if headings.empty?
+        result[:toc] =
+          if headings.empty?
+            I18n.t(:label_wiki_toc_empty)
+          else
+            content_tag(:ul, render_nested(nil), class: 'toc')
+          end
 
-        result[:toc] = content_tag(:ul, render_nested(nil), class: 'toc')
       rescue StandardError => e
         Rails.logger.error { "Failed to render table of contents: #{e} #{e.message}" }
       end