Why forcing empty strings to be undefined when validating via validationSchema?

[velosipedist]

Any empty string is forced to be undefined before starting Yup validation under the hood:

export function prepareDataForValidation<T extends FormikValues>(//...
//...
 // every node is normalized like this before validation
 data[key] = values[key] !== '' ? values[key] : undefined;
//

This means, I cannot be sure that whatever of the values that typed as a string could contain a string. This side-effect is not reflected in typing, so while I am validating something with yup.test, I have to manully type the incoming value like this:

type EmptiableString<T> = T extends object
  ? { [k in keyof T]: EmptiableString<T[k]> }
  : (T extends string ? (T | undefined) : T);

type SomeValueType = string | {
  foo: string
};

.test("isFoo", "contains valid foo", function (this: TestContext, value?: EmptiableString<SomeValueType>) {
  if(value.substring(0) === ""){
    // ^ invalid case that crashes the validation unless I added EmptiableString wrap
    // because empty string silently becomes undefined here
  }

  if(value.foo.toLowerCase() === "bar"){
      // ^ gonna crash without the wrapping, in case there is { foo: "" } in the form
  }
}

Wouldn't that make sense to either remove the undefined forcing or add this kind of type wrapping on test callbacks?