utility to combine PaymentTransactionInfoPlugins

Maybe the pull request can be simplified to hide internal changes that aren't interesting after the fact
Oh, right, I see it now. I would simplify it indeed: if `t2` is `nil`, return `t1` and if `t1` is `nil`, return `t2`. In both cases, if the other one is `nil` too, we would return `nil`, which is fine.