Improve typing when attribute are not nullable

[LouisPinsard]

Hello again!

Currently even with the nulls attribute set to false, the return type of all model methods indicates that non-nullable attribute is: AttributeType | undefined | null.

Would be great to have AttributeType | undefined in those cases.

The idea would be to have :
type Entity<T extends OneModel> = Flatten<Merge<Required<T>, Optional<T>>> instead of type Entity<T extends OneModel> = Flatten<Merge<Required<T>, OptionalOrNull<T>>>

wdyt? happy to help if I can!

[mobsense]

Sounds good.

Again, a PR would be great.

I'm looking for someone with stronger TS skills than mine to help contribute on these tasks.

Soon, I'd actually like to rewrite the code into TS directly also -- but that is a separate task.