Backup restoration asking for password, despite never setting a password.

I recently upgraded from my iPhone 7 to a 12. In preparation for this I backed up my old phone to iTunes on my new laptop. When I went to restore the backup to my new phone it says I need to enter a password for the backup. The only problem is I NEVER set a passcode for this backup. I was not prompted to do so, and I would have remembered doing so. (On top of this, had I set a password for this backup I always reuse a few passwords, so I would have eventually guessed the right one had I forgotten.) A few days before all this I tried to do an iCloud backup (twice) and each time the backup would fail, (this is another problem Apple should address), and I guess this backup saved my contacts and some (not all) of my messages and photos. How do I go about recovering this password? I don't have the old phone anymore as I traded it in and factory reset it, so I can't just do another, unencrypted backup. If anyone has any ideas please send them my way.