pgsql: Check we don't misoptimize a NOT IN where the subquery returnsn

Check we don't misoptimize a NOT IN where the subquery returns no rows.

Future-proofing against a common mistake in attempts to optimize NOT IN.
We don't have such an optimization right now, but attempts to do so
are in the works, and some of 'em are buggy.  Add a regression test case
covering the point.

David Rowley

Discussion: https://postgr.es/m/CAKJS1f90E9agVZryVyUpbHQbjTt5ExqS2Fsodmt5_A7E_cEyVA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3396138a6de329fa7b5e0dda79219b4ae82622dc

Modified Files
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src/test/regress/expected/subselect.out | 13 +++++++++++++
src/test/regress/sql/subselect.sql      |  9 +++++++++
2 files changed, 22 insertions(+)