[hotfix-36382][core] 修复null无法转换为timestamp异常。

Author: simenliuxing

core/src/main/java/com/dtstack/flink/sql/format/dtnest/DtNestRowDeserializationSchema.java

@@ -199,7 +199,7 @@ private Row convertTopRow() {
                 JsonNode node = getIgnoreCase(fieldNames[i]);
                 AbstractTableInfo.FieldExtraInfo fieldExtraInfo = fieldExtraInfos.get(i);
 
-                if (node == null) {
+                if (node == null || node instanceof NullNode) {
                     if (fieldExtraInfo != null && fieldExtraInfo.getNotNull()) {
                         throw new IllegalStateException("Failed to find field with name '"
                                 + fieldNames[i] + "'.");