pgsql: Fix MERGE with DO NOTHING actions into a partitioned table.

Fix MERGE with DO NOTHING actions into a partitioned table.

ExecInitPartitionInfo() duplicates much of the logic in
ExecInitMerge(), except that it failed to handle DO NOTHING
actions. This would cause an "unknown action in MERGE WHEN clause"
error if a MERGE with any DO NOTHING actions attempted to insert into
a partition not already initialised by ExecInitModifyTable().

Bug: #18871
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Author: Tender Wang <tndrwang@gmail.com>
Reviewed-by: Gurjeet Singh <gurjeet@singh.im>
Discussion: https://postgr.es/m/18871-b44e3c96de3bd2e8%40postgresql.org
Backpatch-through: 15

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/14a33d3f0ae6dcbe9b91d33f64b795c2aef6a870

Modified Files
--------------
src/backend/executor/execPartition.c   |  4 +++-
src/backend/executor/nodeModifyTable.c |  2 +-
src/test/regress/expected/merge.out    | 17 +++++++++++++++++
src/test/regress/sql/merge.sql         | 13 +++++++++++++
4 files changed, 34 insertions(+), 2 deletions(-)