pgUnionAll.single() throws an error when the list is empty
### Summary

I have a union field which uses pgUnionAll in the plan. The field returns only one item, so I use `.single()` to get only item from the list returned from pgUnionAll.

### Steps to reproduce

1. Checkout [this branch](https://github.com/achintha-weerasinghe/ouch-my-finger/tree/bug/pg-union-single) from my forked repo of ouch-my-finger.
2. Create a database called `ouch_my_finger` and schema called `public`.
3. Use the db.sql to update the database with the tables and types.
4. Run `yarn dev` to start the dev server.
5. Try querying the `animalsById` with an invalid uuid.

### Expected results

Instead the error, null should be returned.

### Actual results

An error is thrown `TypeError: Cannot read properties of undefined (reading '0')`
