Make PartitionedOutputOperator.finish() idempotent

[losipiuk]

The PartitionedOutputOperator.finish() was not idempotent in the case when only position counter was tracked in
PositionsAppenderPageBuilder. This could result in wrong query results.

Description

Non-technical explanation

Release notes

( ) This is not user-visible and no release notes are required.
(x) Release notes are required, please propose a release note for me.
( ) Release notes are required, with the following suggested text:

# General
* Fix bug which could manifest in wrong results for queries where all data scanned from tables 
  was processed in the leaf stage and was not passed for downstream. ({issue}`issuenumber`)

[lukasz-stec]

lgtm % minor comment

[lukasz-stec]

maybe drop the if instead?

[lukasz-stec]

It would be good to have a test for this case (I can add it in a separate PR). Do you know what kind of query has zero-column partitioning?

[losipiuk]

It would be good to have a test for this case (I can add it in a separate PR). Do you know what kind of query has zero-column partitioning?

I observed that with queries like this one:

SELECT o1.orderkey FROM orders o1 RIGHT JOIN (SELECT * FROM orders LIMIT 10) o2 ON true ORDER BY o1.orderkey LIMIT 20

where you are joining two tables, where neither join condition, nor output depend on any column from one of the tables.

As for the testing, it is problematic as I only observed problem when POO.finish() was called twice. And this does not happen often.