pgsql: Fix new assertion for MERGE view_name ... DO NOTHING.

Fix new assertion for MERGE view_name ... DO NOTHING.

Such queries don't expand automatically updatable views, and ModifyTable
uses the wholerow attribute unconditionally.  The user-visible behavior
is fine, so change to more-specific assertions.  Commit
d5f788b41dc2cbdde6e7694c70dda54d829a5ed5 added the wrong assertion.
Back-patch to v17, where commit 5f2e179bd31e5f5803005101eb12a8d7bf8db8f3
introduced MERGE view_name.

Reported by Alexander Lakhin.

Discussion: https://postgr.es/m/e4b40a88-c134-6926-3196-bc4501cb87a2@gmail.com

Branch
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REL_17_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/0d3b35c367c21fcfe437b689919bf153f85e2e25

Modified Files
--------------
src/backend/executor/nodeModifyTable.c        | 40 +++++++++++++++------------
src/test/regress/expected/updatable_views.out |  5 ++++
src/test/regress/sql/updatable_views.sql      |  5 ++++
3 files changed, 33 insertions(+), 17 deletions(-)