Panic in Spark's `format_string`

### Describe the bug

With the `datafusion-spark` crate enabled:
```
SELECT format_string('Char: %c', -1);
--> External error: task 17 panicked with message "called `Option::unwrap()` on a `None` value"
```

### To Reproduce

_No response_

### Expected behavior

_No response_

### Additional context

_No response_