Prior 7.1 Rails always included `primary_key` in `RETURNING` clause on
record creation. This was changed in 7.1 to include more auto-populated
columns if such columns exist. This change lead to situations where
no columns were requested in `RETURNING` clause, even the `primary_key`.

This change brings back the old behavior of always requesting the
`primary_key` in `RETURNING` clause if no other columns are requested.