[fix](Nereids) fix explain plan with sql cache (#39431) (#39463)

introduced by #38950, explain plan with sql cache will throw an exception
```
errCode = 2, detailMessage = Cannot invoke "org.apache.doris.nereids.trees.plans.Plan.treeString()" because "this.optimizedPlan" is null
```

fe/fe-core/src/main/java/org/apache/doris/nereids/NereidsPlanner.java

@@ -174,7 +174,7 @@ public class NereidsPlanner extends Planner {
             if (plan instanceof LogicalSqlCache) {
                 rewrittenPlan = analyzedPlan = plan;
                 LogicalSqlCache logicalSqlCache = (LogicalSqlCache) plan;
-                physicalPlan = new PhysicalSqlCache(
+                optimizedPlan = physicalPlan = new PhysicalSqlCache(
                         logicalSqlCache.getQueryId(),
                         logicalSqlCache.getColumnLabels(), logicalSqlCache.getFieldInfos(),
                         logicalSqlCache.getResultExprs(), logicalSqlCache.getResultSetInFe(),

regression-test/suites/query_p0/cache/sql_cache.groovy

@@ -212,4 +212,11 @@ suite("sql_cache") {
                 """
 
     sql  "ADMIN SET FRONTEND CONFIG ('cache_last_version_interval_second' = '10')"
+
+    // explain plan with sql cache
+    connect {
+        sql "set enable_sql_cache=true"
+        sql "select 100"
+        sql "explain plan select 100"
+    }
 }