[ BUG ] - "Failed to validate prompt for output" when group node is connected to 'old' reroute

### Prerequisites

- [x] I am running the latest version of ComfyUI
- [x] I have searched existing issues to make sure this isn't a duplicate
- [x] I have tested with all custom nodes disabled ([see how](https://docs.comfy.org/troubleshooting/custom-node-issues#step-1%3A-test-with-all-custom-nodes-disabled))

### What happened?

Group nodes fail when the input is connected to an old 'reroute' node:

### Steps to Reproduce

To reproduce, run the workflow above. The 3rd branch fails because the group node is connected to an 'old' reroute.

### How is this affecting you?

Workflow won't execute

### ComfyUI Frontend Version

1.26.7

### Browser

Chrome/Chromium

### Console Errors

```javascript
Failed to validate prompt for output 18:
* ImageUpscaleWithModel 10:1:
- Required input is missing: image
Output will be ignored
```

### Logs

```shell

```

### Additional Context

_No response_

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-5294-BUG-Failed-to-validate-prompt-for-output-when-group-node-is-connected-to-old--2616d73d36508122a67ffa6b50528a1c) by [Unito](https://www.unito.io)