fix(runtime-core): merge return value when there is undefined value in withDefaults

[godxiaoji]

From #7619, increase the judgment of undefined value in withDefaults.

At the same time, according to the requirements of the document, ensures the returned props type has the optional flags removed for properties.

const res = withDefaults(
  defineProps<{
    str?: string;
    bool: boolean;
  }>(),
  {
    str: undefined,
    bool: undefined
  }
);

// res.str: string | undefined
// res.bool: boolean | undefined