Fix NPE of RelationalDeleteDataNode merge method (#14834)

* done

* done

* fix IT

* fix IT

Author: liyuheng55555

integration-test/src/test/java/org/apache/iotdb/relational/it/db/it/IoTDBInsertTableIT.java

@@ -877,7 +877,7 @@ public void testInsertKeyword() throws IoTDBConnectionException, StatementExecut
   }
 
   @Test
-  public void testInsertSingleColumn() throws SQLException {
+  public void testInsertSingleColumn() throws SQLException, InterruptedException {
     try (Connection connection = EnvFactory.getEnv().getConnection(BaseEnv.TABLE_SQL_DIALECT);
         Statement st1 = connection.createStatement()) {
       st1.execute("use \"test\"");
@@ -893,8 +893,12 @@ public void testInsertSingleColumn() throws SQLException {
             "305: [INTERNAL_SERVER_ERROR(305)] Exception occurred: \"insert into sg21(time) values(1)\". executeStatement failed. No column other than Time present, please check the request",
             e.getMessage());
       }
+      // sleep a while to avoid the same timestamp between two insertions
+      Thread.sleep(10);
       // only attribute
       st1.execute("insert into sg21(ss1) values('1')");
+      // sleep a while to avoid the same timestamp between two insertions
+      Thread.sleep(10);
       // only field
       st1.execute("insert into sg21(ss2) values(1)");
 

iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/planner/plan/node/write/RelationalDeleteDataNode.java

@@ -318,7 +318,9 @@ public SearchNode merge(List<SearchNode> searchNodes) {
     if (relationalDeleteDataNodeList.stream()
         .anyMatch(
             relationalDeleteDataNode ->
-                !this.getDatabaseName().equals(relationalDeleteDataNode.getDatabaseName()))) {
+                this.getDatabaseName() != null
+                    && !this.getDatabaseName()
+                        .equals(relationalDeleteDataNode.getDatabaseName()))) {
       throw new IllegalArgumentException("All database name need to be same");
     }
     List<TableDeletionEntry> allTableDeletionEntries =