want to narrow type in handleSubmit function's (data) prop!!

[al-bur]

below is it's just pseudo code!!
plz just focus on logic

import React from 'react';
import { useForm, SubmitHandler } from 'react-hook-form';
import { zodResolver } from '@hookform/resolvers/zod';
import * as z from 'zod';

type FormData = { name?: string }

const MyForm: React.FC = () => {

  const { register, handleSubmit, formState: { errors } } = useForm<FormData>({
     defaultValue: { name: undefined} // it's just pseudo code
  });

  **// HERE IS THE QUESTION!!**
  const onSubmit: SubmitHandler<FormData> = (data) => {
     **// data type is string | undefined**

     **Q. i want to narrow type like data.name which is string(not optional)?? because RHF controller already checks required type with rules!!** => but recent version's RHF's data.name type is string | undefined
  };

  return (
    <form onSubmit={handleSubmit(onSubmit)}>
      <div>
        <label>Name</label>
        <input {...register('name') + rules: { required: true } />
        {errors.name && <p>{errors.name.message}</p>}
      </div>
      <button type="submit">Submit</button>
    </form>
  );
};

export default MyForm;

Q. i want to narrow type like data.name which is string(not optional)??
because RHF controller already checks required type with rules!! but recent version's RHF's data.name type is string | undefined