coalesce(Int, Utf8) coerces to Int

### Describe the bug

Type coercion of `COALESCE` returns Int even when there are string arguments. This is not always correct, as '' is a valid string but cannot be converted to int.

### To Reproduce

```sql
SELECT coalesce(1,'')
```

should be typed as a string, not int

### Expected behavior

_No response_

### Additional context

_No response_

Start by running the reported SQL reproduction, SELECT coalesce(1,''), and inspect how COALESCE determines its result type. Compare the inferred type with the expected string type and add or update coverage for this case; done means string arguments are not incorrectly coerced to Int.

Written by the indexing model from the issue text.