[5.x] Double check if structure exists in locale

[Jade-GG]

When saving an entry that only exists in some locales, Statamic will throw an error because the structure doesn't exist.

I'm not sure if there are specific conditions that need to be met for this to be reproducible, but I can reproduce it every time in my local project by removing one of the locales from my pages collection and then trying to save a random entry.