release-23.2: plpgsql: don't exit early when SELECT INTO returns no rows

[blathers-crl]

Backport 1/1 commits from #115650 on behalf of @DrewKimball.

/cc @cockroachdb/release

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Previously, a SELECT INTO statement that returned zero rows wouldn't call the continuation, since the continuation is called as a projection. This patch adds a RIGHT JOIN with a zero-column one-row Values operator to ensure that SELECT INTO statements are correctly null-extended when the underlying SQL statement returns no rows. See the postgres docs:

https://www.postgresql.org/docs/16/plpgsql-statements.html#PLPGSQL-STATEMENTS-SQL-ONEROW

Fixes #114826

Release note (bug fix): Fixed a bug that existed only in pre-release versions v23.2.0-beta.1 and v23.2.0-beta.2 which could cause PLpgSQL routines with SELECT INTO syntax to return early.

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Release justification: bug fix for new functionality in 23.2

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
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[cockroach-teamcity]

This change is 

[mgartner]

Reviewed 10 of 10 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @DrewKimball and @rharding6373)

[yuzefovich]

Merging since backport of #115657 needs to be on top of this.