What's the condition for DocumentLink target to be undefined?

I would like to use api to get links from the document:

```ts
const links = await vscode.commands.executeCommand<DocumentLink[]>('vscode.executeLinkProvider', document.uri);
```

`DocumentLink` is guaranteed to have a `range: Range;`, but not the `target?: Uri;`. What are the cases when `target` is undefined, or am I using the wrong type?