update src/gausskernel/optimizer/plan/createplan.cpp.

修改create_append_plan函数中对 path为subquery且bestpath->subpaths为空 的情况的处理
参考Gaussdb和Postgres中处理方式，将原有的断言修改为if判断。
当出现上述情况是判断subplan为dummy plan，是则继续执行make_subqueryscan;反之则执行make_result.

src/gausskernel/optimizer/plan/createplan.cpp

@@ -1072,8 +1072,8 @@ static Plan* create_append_plan(PlannerInfo* root, AppendPath* best_path)
          * subquery's rangetable so that the executor will apply permissions
          * checks to those rels at runtime.)
          */
-        if (rel->rtekind == RTE_SUBQUERY) {
-            Assert(is_dummy_plan(rel->subplan));
+        if (rel->rtekind == RTE_SUBQUERY &&
+            (rel->subplan != NULL && is_dummy_plan(rel->subplan) == true)) {
             return (Plan*)make_subqueryscan(tlist, NIL, rel->relid, rel->subplan);
         } else {
             /* Generate a Result plan with constant-FALSE gating qual */