Calling `load_async` on a null relation was executing the fallback
`WHERE 1=0` query, since it doesn't go through `exec_queries`.

Since 2a901049, `exec_queries` is
implemented in terms of `exec_main_query`, so we only need to override
that one method.

We can also stop assigning `@records`, as that now happens in `load`
since 847643a5.