Error on v2 on-the-spot registration page

[dunkOnIT]

Reported on Discord:

Previously for on-the-spot reg for a returning competitor, you could put in just name and WCA ID and it would work. For a comp beta testing the new registration system, we just get undefined method 'iso2' for nil doing that, so we had to re-get all the competitor's info (birthday/email/etc.) which then works but is a bit annoying.

@FinnIckler any chance you'd be able to look into this when you have a moment? If it's not a quick fix for you, I'm also happy to investigate

[FinnIckler]

ehhh... I didn't actually touch that part of the code if you look at that PR https://github.com/thewca/worldcubeassociation.org/pull/9948/files