[BUG] in vision-annotate-images: Unexpected token o in JSON at position 1

### Which sample has a bug?

**vision-annotate-images**

### How to reproduce the issue

Deploy the function as documented, then call it from Swift, it returns the following error:

### Debug output

```
Optional(__C.FIRFunctionsErrorCode)
Unexpected token o in JSON at position 1
```

**Errors in the
[console logs](https://console.firebase.google.com/project/_/functions/logs?search=&severity=DEBUG)**

**Screenshots**

### Expected behavior

The function should return a json string containing the text boxes.

### Actual behavior

It crashed and returned nil.