types(defineComponent): fix unwrap when returning Ref<T>|undefined from setup

[pikax]

fix #3990

ShallowUnwrapRef was not handling the Ref<T>|undefined type

Minimal repro

defineComponent({
  setup() {
    return {
      a: undefined as Ref<number> | undefined
    }
  },
  methods: {
    test() {
      // @ts-expect-error `a` should not be a ref
      this.a?.value
    }
  }
})