Fixes https://github.com/rubysherpas/paranoia/issues/274

It looks like ActiveRecord refactored the Transaction class hierarchy in
a non-backwards-compatible
way (https://github.com/rails/rails/pull/16363), which breaks
`add_to_transaction` for null transactions (I think it probably
qualifies as a bug). The workaround is to not try to use
`add_to_transaction` in null transactions.