ReferenceError when resource has no functions

Having an empty resource:
```wit
interface execution {

resource join-set-id {
// id: func() -> string;
}

new-join-set: func() -> join-set-id;
}
```
Calling `new-join-set` from a JavaScript guest fails with an error: `ReferenceError: import_execution_1_0_0$JoinSetId is not defined`.
This is because the generated `initializer.js`does not contain `class import_execution_1_0_0$JoinSetId {...}`, but tries to use it in the generated `new-join-set` function: `var rsc2 = new.target === import_execution_1_0_0$JoinSetId ? this : Object.create(import_execution_1_0_0$JoinSetId.prototype);`

The workaround is to add a function, which triggers the `class` generation.
Tested with componentize-js version 0.18.2 .

使用 componentize-js 0.18.2 重现空资源的 WIT 示例，并检查生成的 initializer.js。跟踪生成的 new-join-set 函数，并验证空资源会生成其 JoinSetId 类，以及从 JavaScript guest 调用时不再引发 ReferenceError。

由索引模型根据 Issue 内容生成。