`export {}` was removed incorrectly in the module without export

tsup version: `6.6.3`

1. Write a module without export

```
// center.ts
declare global {
interface String {
center(width: number, fillChar?: string): string;
}
}

Sring.prototype.center=function (width:number,fillChar=" ") {
let s=this
let llength=Math.floor((width-s.length)/2)
return new Array(llength).join(fillChar)+s+new Array(width-s.length-llength).join(fillChar)
}
export {} // necessary
```
2. tsup.config.js

```
import { defineConfig } from 'tsup'

export default defineConfig({
entry: [
'src/*.ts'
],
format: ['esm','cjs'],
dts: true,
splitting: true,
sourcemap: false,
clean: true,
treeshake:true,
minify: true,
```

3. build result

// center.d.ts
```
declare global {
interface String {
center(width: number, fillChar?: string): string;
}
}
```

`export {}` removed,causes `String.prototype.center` to be unrecognized。

I have to use a hacker method:

```
// center.ts
declare global {
interface String {
center(width: number, fillChar?: string): string;
}
}

Sring.prototype.center=function (width:number,fillChar=" ") {
let s=this
let llength=Math.floor((width-s.length)/2)
return new Array(llength).join(fillChar)+s+new Array(width-s.length-llength).join(fillChar)
}
export const _=0 // replace for export {}
```

Reproduce the issue with center.ts and tsup.config.js, then compare the generated center.d.ts with the source declaration. Check how the declaration build handles the standalone export {}. Done means the generated declaration preserves module scope so the global String augmentation is recognized without a dummy export.

Written by the indexing model from the issue text.