[fix](sql block) throw npe when use Nereids explain or fallback

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

@@ -81,7 +81,7 @@ public class NereidsPlanner extends Planner {
     public static final Logger LOG = LogManager.getLogger(NereidsPlanner.class);
     private CascadesContext cascadesContext;
     private final StatementContext statementContext;
-    private List<ScanNode> scanNodeList = null;
+    private final List<ScanNode> scanNodeList = Lists.newArrayList();
     private DescriptorTable descTable;
 
     private Plan parsedPlan;
@@ -132,7 +132,7 @@ public void plan(StatementBase queryStmt, org.apache.doris.thrift.TQueryOptions
         }
         PlanFragment root = physicalPlanTranslator.translatePlan(physicalPlan);
 
-        scanNodeList = planTranslatorContext.getScanNodes();
+        scanNodeList.addAll(planTranslatorContext.getScanNodes());
         descTable = planTranslatorContext.getDescTable();
         fragments = new ArrayList<>(planTranslatorContext.getPlanFragments());
         for (int seq = 0; seq < fragments.size(); seq++) {

fe/fe-core/src/main/java/org/apache/doris/qe/StmtExecutor.java

@@ -2569,6 +2569,7 @@ public List<ResultRow> executeInternalQuery() {
                     } catch (Exception e) {
                         LOG.warn("fall back to legacy planner, because: {}", e.getMessage(), e);
                         parsedStmt = null;
+                        planner = null;
                         context.getState().setNereids(false);
                         analyzer = new Analyzer(context.getEnv(), context);
                         analyze(context.getSessionVariable().toThrift());

regression-test/suites/sql_block_rule_p0/test_sql_block_rule.groovy

@@ -174,25 +174,31 @@ suite("test_sql_block_rule") {
         "enable"="true");
     """
 
-    test {
-        sql("""SELECT * FROM a_partitioned_table_for_sql_block_rule;""", false)
-
-        exception """sql hits sql block rule"""
-
+    try {
+        test {
+            sql("""SELECT * FROM a_partitioned_table_for_sql_block_rule;""", false)
+            exception """sql hits sql block rule"""
+        }
+    } finally {
+        sql """
+            drop SQL_BLOCK_RULE if exists test_rule_partition;
+        """
     }
 
     sql """
         CREATE SQL_BLOCK_RULE if not exists test_rule_tablet PROPERTIES ( "tablet_num" = "3", "global" = "true",
         "enable"="true");
     """
-
-    test {
-        sql("""SELECT * FROM a_partitioned_table_for_sql_block_rule;""", false)
-
-        exception """sql hits sql block rule"""
-
+    try {
+        test {
+            sql("""SELECT * FROM a_partitioned_table_for_sql_block_rule;""", false)
+            exception """sql hits sql block rule"""
+        }
+    } finally {
+        sql """
+            drop SQL_BLOCK_RULE if exists test_rule_tablet;
+        """
     }
 
 
-
 }