pgsql: Fix crash in postgres_fdw for provably-empty remote UPDATE/DELET

Fix crash in postgres_fdw for provably-empty remote UPDATE/DELETE.

In 86dc90056, I'd written find_modifytable_subplan with the assumption
that if the immediate child of a ModifyTable is a Result, it must be
a projecting Result with a subplan. However, if the UPDATE or DELETE
has a provably-constant-false WHERE clause, that's not so: we'll
generate a dummy subplan with a childless Result. Add the missing
null-check so we don't crash on such cases.

Per report from Alexander Pyhalov.

Discussion: https://postgr.es/m/b9a6f53549456b2f3e2fd150dcd79d72@postgrespro.ru

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b9734c13f168ef0d487aa122e486ca9b6dd6aa59

Modified Files
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contrib/postgres_fdw/expected/postgres_fdw.out | 20 ++++++++++++++++++++
contrib/postgres_fdw/postgres_fdw.c | 4 +++-
contrib/postgres_fdw/sql/postgres_fdw.sql | 4 ++++
3 files changed, 27 insertions(+), 1 deletion(-)