code splitting build generates "empty" source map which causes a warning on Firefox

Reproduction: [esbuild try](https://esbuild.github.io/try/#YgAwLjI0LjAAewogIGVudHJ5UG9pbnRzOiBbImVudHJ5MS5qcyIsICJlbnRyeTIuanMiXSwKICBmb3JtYXQ6ICdlc20nLAogIGJ1bmRsZTogdHJ1ZSwKICBzcGxpdHRpbmc6IHRydWUsCiAgc291cmNlbWFwOiB0cnVlLAogIG91dGRpcjogJ2Rpc3QnLAp9AGUAZW50cnkxLmpzAGV4cG9ydCBjb25zdCBlbnRyeTEgPSAiZW50cnkxIgAAZW50cnkyLmpzAGV4cG9ydCBjb25zdCBlbnRyeTIgPSAiZW50cnkyIgpleHBvcnQgeyBlbnRyeTEgfSBmcm9tICIuL2VudHJ5MS5qcyI)
Related issue on Vite: https://github.com/vitejs/vite/issues/17474

As seen in the reproduction, when code splitting build outputs an entry which only re-exports from a common chunk, it generates an "empty" source map.

- `dist/entry1.js`

```js
import {
entry1
} from "./chunk-PTJ7TEWR.js";
export {
entry1
};
//# sourceMappingURL=entry1.js.map
```

- `dist/entry1.js.map`

```json
{
"version": 3,
"sources": [],
"sourcesContent": [],
"mappings": "",
"names": []
}
```

I thought this behavior seems fine, but it looks like Firefox gives a warning in devtool console https://firefox-source-docs.mozilla.org/devtools-user/debugger/source_map_errors

```
Source map error: No sources are declared in this source map.
Resource URL: http://localhost:5173/node_modules/.vite/deps/preact.js?v=45e47ff9
Source Map URL: preact.js.map [Learn More]
```

Maybe this is something Firefox should tolerate, but I tested `rolldown` with a similar setup and found that it doesn't generate a source map for `entry1.js` for this specific case https://stackblitz.com/edit/vitejs-vite-npr539?file=dist%2Frolldown%2Fentry1.js, so I thought this could be handled on esbuild similarly.