when failing to get AllowedPostTitleFormattingTypes, PostTitleScrubber should use the default allow list

Author: Oaphi

app/helpers/posts_helper.rb

@@ -91,7 +91,7 @@ def initialize
       attrs = []
       tags = []
 
-      allowed_types = SiteSetting['AllowedPostTitleFormattingTypes'] || []
+      allowed_types = SiteSetting['AllowedPostTitleFormattingTypes'] || ['code', 'italic', 'keyboard']
       tags.push('del', 'strike') if allowed_types.include?('strikethrough')
       tags << 'code' if allowed_types.include?('code')
       tags << 'kbd' if allowed_types.include?('keyboard')