BUG #19462: MERGE on partitioned table with INSERT DEFAULT VALUES fails with "iso-8859-1 action in MERGE WHEN clau

The following bug has been logged on the website:

Bug reference:      19462
Logged by:          chunling qin
Email address:      303677365@qq.com
PostgreSQL version: 15.0
Operating system:   centos
Description:

## Summary

A MERGE statement targeting a partitioned table fails with `ERROR: unknown
action in MERGE WHEN clause` when the NOT MATCHED clause uses `INSERT
DEFAULT VALUES`.

## PostgreSQL Version

```
PostgreSQL 15devel on x86_64-pc-linux-gnu, compiled by clang version 17.0.6,
64-bit
```

Tested on REL_15_STABLE branch.

## Steps to Reproduce

```sql
-- Setup: Create partitioned target table
CREATE TABLE pmerge_tgt (id INT) PARTITION BY RANGE (id);
CREATE TABLE pmerge_tgt_lo PARTITION OF pmerge_tgt FOR VALUES FROM
(MINVALUE) TO (100);
CREATE TABLE pmerge_tgt_hi PARTITION OF pmerge_tgt FOR VALUES FROM (100) TO
(MAXVALUE);
CREATE TABLE pmerge_tgt_def PARTITION OF pmerge_tgt DEFAULT;

-- Create source table
CREATE TABLE pmerge_src (id INT);

-- Insert test data
INSERT INTO pmerge_tgt VALUES (50);
INSERT INTO pmerge_src VALUES (200);

-- Bug reproduction
MERGE INTO pmerge_tgt t USING pmerge_src s ON t.id > 1000
  WHEN MATCHED THEN DO NOTHING
  WHEN NOT MATCHED THEN INSERT DEFAULT VALUES;
```
This issue could not be reproduced with the latest version, but we can
easily reproduced  by  PostgreSQL 15devel. I am submitting this report for
official confirmation.

-- Bug reproduction
MERGE INTO pmerge_tgt t USING pmerge_src s ON t.id > 1000
  WHEN MATCHED THEN DO NOTHING
  WHEN NOT MATCHED THEN INSERT DEFAULT VALUES;ERROR:  relation "pmerge_tgt"
already exists
postgres=# ERROR:  relation "pmerge_tgt_lo" already exists
postgres=# ERROR:  relation "pmerge_tgt_hi" already exists
postgres=# ERROR:  relation "pmerge_tgt_def" already exists
postgres=# postgres=# postgres=# ERROR:  relation "pmerge_src" already
exists
postgres=# postgres=# postgres=# INSERT 0 1
postgres=# INSERT 0 1
postgres=# postgres=# postgres=# postgres-# postgres-#
ERROR:  unknown action in MERGE WHEN clause