Directly querying user defined function returning optional fails typecheck

Given a schema:
```edgeql
function MaybeInt() -> optional int64 using (1);
```

Trying to query a call to the function:
```py
result = self.client.query(default.MaybeInt())
```

Fails typechecking with the error:
```
error: Argument 1 to "query" of "ReadOnlyExecutor" has incompatible type "type[int64] | None"; expected "QueryableObject[int] | type[QueryableType[int]]" [arg-type]
```

Note, the query correctly returns `[1]`.