Incorrect nullability in some result set schemas

Due to an omission in result set schema checking in engine tests, there are several queries in engine tests that have an incorrect nullability declared in test expectations or returned.

Example query:

```sql
select sum(1) as s from emptytable
```

The result schema for this query is declared as non-nullable, but this query returns a single null result (correctly).

Due to this oversight in the testing framework, there are many test queries that fail once we add it. We need to test against MySQL and verify which of these queries return nullable result schemas.

Here's a PR with the missing nullability check added.

https://github.com/dolthub/go-mysql-server/pull/3271