If your data should not be successfully folded if it's empty, you should've already parsed it as an Optional nonEmptyList instead of letting illegal states fly around for a while in your application.
Yeah that's good application design, but those concerns aren't so relevant to the person writing the standard library for a language. You can certainly include a specialized version of reduce for nonEmptyLists which just returns A, but that doesn't change the fact that you have to return an Optional for a normal possibly-empty iterator if you want your reduce function to be non-partial.
I.e. there is no initial value to pass in, but the result is an Optional to handle the empty iterator case. That’s how rust does it, for example:
https://doc.rust-lang.org/std/iter/trait.Iterator.html#metho...