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

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -6853,6 +6853,26 @@ DROP TRIGGER trig_row_before ON rem1;
 DROP TRIGGER trig_row_after ON rem1;
 DROP TRIGGER trig_local_before ON loc1;
 -- Test direct foreign table modification functionality
+EXPLAIN (verbose, costs off)
+DELETE FROM rem1;                 -- can be pushed down
+                 QUERY PLAN                  
+---------------------------------------------
+ Delete on public.rem1
+   ->  Foreign Delete on public.rem1
+         Remote SQL: DELETE FROM public.loc1
+(3 rows)
+
+EXPLAIN (verbose, costs off)
+DELETE FROM rem1 WHERE false;     -- currently can't be pushed down
+                      QUERY PLAN                       
+-------------------------------------------------------
+ Delete on public.rem1
+   Remote SQL: DELETE FROM public.loc1 WHERE ctid = $1
+   ->  Result
+         Output: ctid
+         One-Time Filter: false
+(5 rows)
+
 -- Test with statement-level triggers
 CREATE TRIGGER trig_stmt_before
 	BEFORE DELETE OR INSERT OR UPDATE ON rem1