Ensure Validated<T> honors nullability of parameter like the default binding does

[DamianEdwards]

Currently, Validated<TValue> can return a null value even if the parameter was declared as non-nullable. RequestDelegateFactory (i.e. the default binding logic in the framework) honors the nullability declared on the parameter type and in the case it determines nullability is not allowed and a null value is found, auto-replies with a 400 Bad Request. Validator<TValue> can't auto-reply but it can set add an error to the errors collection and set a status code.

This might already be working as intended and best case is it just needs a test to verify.