Allow ActiveStorage::Attachment creation with no record attachments

FIxes rails#51882

Author: justinko

activestorage/app/models/active_storage/attachment.rb

@@ -156,7 +156,7 @@ def dependent
     end
 
     def named_variants
-      record.attachment_reflections[name]&.named_variants
+      record.attachment_reflections[name]&.named_variants || {}
     end
 
     def transformations_by_name(transformations)