pgsql: Fix bogus ctid requirement for dummy-root partitioned targets

Fix bogus ctid requirement for dummy-root partitioned targets

ExecInitModifyTable() unconditionally required a ctid junk column even
when the target was a partitioned table. This led to spurious "could
not find junk ctid column" errors when all children were excluded and
only the dummy root result relation remained.

A partitioned table only appears in the result relations list when all
leaf partitions have been pruned, leaving the dummy root as the sole
entry. Assert this invariant (nrels == 1) and skip the ctid requirement.
Also adjust ExecModifyTable() to tolerate invalid ri_RowIdAttNo for
partitioned tables, which is safe since no rows will be processed in
this case.

Bug: #19099
Reported-by: Alexander Lakhin <exclusion(at)gmail(dot)com>
Author: Amit Langote <amitlangote09(at)gmail(dot)com>
Reviewed-by: Tender Wang <tndrwang(at)gmail(dot)com>
Reviewed-by: Kirill Reshke <reshkekirill(at)gmail(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/19099-e05dcfa022fe553d%40postgresql.org
Backpatch-through: 14

Branch
------
REL_17_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/933f67fb6a79d26c5fd69ca6adbadae9efd215fc

Modified Files
--------------
contrib/file_fdw/expected/file_fdw.out | 15 +++++++++++++++
contrib/file_fdw/sql/file_fdw.sql | 18 ++++++++++++++++++
src/backend/executor/nodeModifyTable.c | 19 ++++++++++++++++---
3 files changed, 49 insertions(+), 3 deletions(-)