pgsql: Fix incorrect pruning of NULL partition for boolean IS NOT claus

Fix incorrect pruning of NULL partition for boolean IS NOT clauses

Partition pruning wrongly assumed that, for a table partitioned on a
boolean column, a clause in the form "boolcol IS NOT false" and "boolcol
IS NOT true" could be inverted to correspondingly become "boolcol IS true"
and "boolcol IS false".  These are not equivalent as the NOT version
matches the opposite boolean value *and* NULLs.  This incorrect assumption
meant that partition pruning pruned away partitions that could contain
NULL values.

Here we fix this by correctly not pruning partitions which could store
NULLs.

To be affected by this, the table must be partitioned by a NULLable boolean
column and queries would have to contain "boolcol IS NOT false" or "boolcol
IS NOT true".  This could result in queries filtering out NULL values
with a LIST partitioned table and "ERROR:  invalid strategy number 0"
for RANGE and HASH partitioned tables.

Reported-by: Alexander Lakhin
Bug: #18344
Discussion: https://postgr.es/m/18344-8d3f00bada6d09c6@postgresql.org
Backpatch-through: 12

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/fb95cc72bfe7a31efa8a711b8094a0c41ffad291

Modified Files
--------------
src/backend/partitioning/partprune.c          |  56 +++++++++++++-
src/test/regress/expected/partition_prune.out | 103 ++++++++++++++++++++++++++
src/test/regress/sql/partition_prune.sql      |  30 ++++++++
3 files changed, 187 insertions(+), 2 deletions(-)