error transfrom from ESM `export default` to CJS `module.exports`

## issue

It will only transform `export default` to `module.exports` when
- the entry file has only `export default` without any `named export`
- with `splitting: true` and `cjsInterop: true` in the `tsup.config.ts`.

In other cases, it doesn't work.

## reproduction

https://github.com/LoTwT/tsup-cjs-default-export-issue

- pnpm i
- pnpm build
- node test.cjs

The result is false.

- remove the `export const fn` in the `index.ts`
- pnpm build
- node test.cjs

The result is true.