Fix tablespace handling in MERGE/SPLIT partition commands.

Fix tablespace handling in MERGE/SPLIT partition commands.

As commit ca4103025d stated, new partitions without a specified tablespace
should inherit the parent relation's tablespace. However, previously,
ALTER TABLE MERGE PARTITIONS and ALTER TABLE SPLIT PARTITION commands
always created new partitions in the default tablespace, ignoring
the parent's tablespace. This commit ensures new partitions inherit
the parent's tablespace.

Backpatch to v17 where these commands were introduced.

Author: Fujii Masao
Reviewed-by: Masahiko Sawada
Discussion: https://postgr.es/m/abaf390b-3320-40a5-8815-ef476db5cfe7@oss.nttdata.com