Misleading error message when request is `""`

### System Info

system unrelated

### Details

```js
import {} from '';

```

rspack will throw error: 
```bash
'should have context', crates/rspack_plugin_externals/src/plugin.rs:126:16

```

### Reproduce link

_No response_

### Reproduce Steps

1. npm create @rspack/cli@latest
2. change index.js to above
3. pnpm build