fix value type on validation result

[mastery-benjamin-palko]

The value property returned from .validate is always any, which forces casting to the expected type.

The value should always be known (TSchema) and the error property will be optional, check if error is falsey to determine if value has been properly validated.

Example:

const { value, error } = <schema>.validate(<obj>);

if (error) {
  console.error(error);
  return;
}

// do stuff...

Related to #2677