Fix CacheBuster being queued for missing media attachments (#34253)

app/models/media_attachment.rb

@@ -420,6 +420,8 @@ class MediaAttachment < ApplicationRecord
 
     @paths_to_cache_bust = MediaAttachment.attachment_definitions.keys.flat_map do |attachment_name|
       attachment = public_send(attachment_name)
+      next if attachment.blank?
+
       styles = DEFAULT_STYLES | attachment.styles.keys
       styles.map { |style| attachment.url(style) }
     end.compact