Query Console has no result when splitting supplementary code points

It appears predicates for the built-in type `string` do not have any result in the Query Console when splitting supplementary Unicode code points into the corresponding surrogate chars.
I am not sure whether the VSCode extension behaves differently, and am not sure either what the expected behavior is.

```ql
select "😊".prefix(1)
```
[Query Console link](https://lgtm.com/query/5938355067463238403/)

Edit: Same also applies when not including the split supplementary code point in the result:
```ql
where exists("😊".prefix(1))
select "hello world"
```

As a side note: Trying to use unpaired surrogates in the Query Console causes "Failed to execute" (cannot provide example code here because GitHub does not preservere unpaired surrogate; and Query Console does not run so cannot provide link either):