Stop emitting empty files

Minimal reproduction -

File structure


tsup.config.ts
```ts
import { defineConfig } from 'tsup'

export default defineConfig({
entry: ['lib', 'types'],
dts: true,
})
```

types/helpers.ts (only contains types)

```ts
export type Optional = T | null | undefined
```

lib/index.ts

```ts
import { Optional } from '../types/helpers'

export function log(message?: Optional) {
console.log(message ?? '')
}
```

This will output helpers.js with empty exports or full empty file if using esm format


Reproduce the issue with the shown tsup.config.ts, types/helpers.ts, and lib/index.ts, then inspect how the two entry points are processed and why a type-only entry produces helpers.js. Compare the generated ESM and other outputs; done means the type-only entry emits no empty JavaScript file while lib/index.ts still builds correctly.

Written by the indexing model from the issue text.