[fix](Nereids) topn two phase read do not process child correct (#19136)

fe/fe-core/src/main/java/org/apache/doris/nereids/processor/post/TwoPhaseReadOpt.java

@@ -92,6 +92,7 @@ public class TwoPhaseReadOpt extends PlanPostProcessor {
         PhysicalOlapScan olapScan;
         PhysicalProject<Plan> project = null;
         PhysicalFilter<Plan> filter = null;
+        child = localTopN.child();
         while (child instanceof Project || child instanceof Filter) {
             if (child instanceof Filter) {
                 filter = (PhysicalFilter<Plan>) child;