Describe the bug

When using #[serde(flatten)] with a zero-field struct that derives TS, the TypeScript export generation panics with the error: "[StructName here] cannot be flattened".

To Reproduce

Steps to reproduce the behavior:

Create a zero-field struct with `TS`:
#[derive(Deserialize, Serialize, TS)]
#[ts(export)]
pub struct Inner {}

// Create a parent struct that flattens the zero-field struct:
#[derive(Deserialize, Serialize, TS)]
#[ts(export)]
pub struct Outer {
    #[serde(flatten)]
    pub inner: Inner,
    pub other_field: String,
}

3. Run TypeScript export
4. See panic: "Inner cannot be flattened"
5. Surprised pikachu

Expected behavior

The zero-field struct should be flattened without issue, resulting in TypeScript output equivalent to:

export type Outer {
    other_field: string;
}

Or at least the library should panic indicating that zero-field structs cannot be flattened.

Version

12.0.1

Workaround:

I could just remove the whole struct and it's flatten.

Why?

That empty struct is a placeholder which could change at any moment I don't want to remove it even though it is empty