broken link references in workflow json are not converted correctly to api workflows gracefully

### Prerequisites

- [x] I am running the latest version of ComfyUI
- [ ] I have custom nodes enabled

### What happened?

two template workflows contain link IDs in their subgraph node data that do not exist in the subgraph's link table. `graphToPrompt` throws `InvalidLinkError` and cannot convert these workflows at all.

### Steps to Reproduce

open and save either of these:
- `image_qwen_Image_2512`: `InvalidLinkError: No link found in parent graph for id [86:3] slot [5] steps`
- `image_qwen_Image_2512_controlnet`: same error

### How is this affecting you?

Crashes ComfyUI completely

### ComfyUI Frontend Version

1.43.1

### Browser

Chrome/Chromium

### Console Errors

```javascript

```

### Logs

```shell

```

### Additional Context

`comfyui-workflow-templates==0.9.27`

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-10259-broken-link-references-in-workflow-json-are-not-converted-correctly-to-api-workflows--3276d73d3650816b8fc7d2c0fbac5fb0) by [Unito](https://www.unito.io)