This fix ensures that `WHERE` conditions are emitted after the `LEFT
JOIN` match flag is set, so rows from the right table are properly
filtered, even when they are `NULL` due to the outer join.
Previously, the query below would return rows where `products.price` was
`NULL`:
```sql
SELECT users.id, price
FROM users
LEFT JOIN products ON users.id = products.id
WHERE products.price IS NOT NULL;
```
Reviewed-by: Jussi Saurio <jussi.saurio@gmail.com>
Closes#2501
cca2f6c947