fix: throw null pointer exception if the set value is null in RowBuilder

docs/en/quickstart/sdk/java_sdk.md

@@ -241,7 +241,8 @@ try {
 
 1. Get `InsertPrepareStatement` by calling `SqlClusterExecutor::getInsertPreparedStmt(db, insertSqlWithPlaceHolder)` interface.
 2. Use `PreparedStatement::setType(index, value)` interface to fill in data to the `InsertPrepareStatement`. Note that the index starts from 1.
-3. Use `PreparedStatement::execute()` interface to execute the insert statement.
+3. For String, Date and Timestamp types, null objects can be set using either `setType(index, null)` or `setNull(index)`.
+4. Use `PreparedStatement::execute()` interface to execute the insert statement.
 
 ```{note}
 When the conditions of the `PreparedStatement` are the same, you can repeatedly call the set method of the same object to fill in data before executing `execute`. There is no need to create a new `PreparedStatement` object.

docs/zh/quickstart/sdk/java_sdk.md

@@ -245,7 +245,8 @@ try {
 
 1. 使用 `SqlClusterExecutor::getInsertPreparedStmt(db, insertSqlWithPlaceHolder)` 接口获取 InsertPrepareStatement。
 2. 调用 `PreparedStatement::setType(index, value)` 接口，填充数据到 InsertPrepareStatement中。注意 index 从 1 开始。
-3. 使用 `PreparedStatement::execute()` 接口执行 insert 语句。
+3. 对于String, Date和Timestamp类型, 可以通过`setType(index, null)`和`setNull(index)`两种方式来设置null对象。 
+4. 使用 `PreparedStatement::execute()` 接口执行 insert 语句。
 ```{note}
 PreparedStatment条件相同时，可以对同一个对象反复set填充数据后，再执行execute，不需要重新创建PreparedStatement。
 ```

java/openmldb-common/src/main/java/com/_4paradigm/openmldb/common/codec/ClassicRowBuilder.java

@@ -205,6 +205,9 @@ public boolean appendTimestamp(Timestamp val) {
         if (!check(DataType.kTimestamp)) {
             return false;
         }
+        if (val == null) {
+            return appendNULL();
+        }
         setField(cnt);
         buf.position(offsetVec.get(cnt));
         buf.putLong(val.getTime());
@@ -250,6 +253,9 @@ public boolean appendDouble(double val) {
 
     @Override
     public boolean appendDate(Date date) {
+        if (date == null) {
+            return appendNULL();
+        }
         int dateInt = CodecUtil.dateToDateInt(date);
         buf.position(offsetVec.get(cnt));
         buf.putInt(dateInt);
@@ -260,11 +266,14 @@ public boolean appendDate(Date date) {
 
     @Override
     public boolean appendString(String val) {
-        byte[] bytes = val.getBytes(CodecUtil.CHARSET);
-        int length = bytes.length;
-        if (val == null || (!check(DataType.kVarchar) && !check(DataType.kString))) {
+        if (!check(DataType.kVarchar) && !check(DataType.kString)) {
             return false;
         }
+        if (val == null) {
+            return appendNULL();
+        }
+        byte[] bytes = val.getBytes(CodecUtil.CHARSET);
+        int length = bytes.length;
         if (strOffset + length > size) {
             return false;
         }

java/openmldb-common/src/main/java/com/_4paradigm/openmldb/common/codec/FlexibleRowBuilder.java

@@ -273,6 +273,9 @@ public boolean setTimestamp(int idx, Timestamp val) {
         if (!checkType(idx, Type.DataType.kTimestamp)) {
             return false;
         }
+        if (val == null) {
+            return setNULL(idx);
+        }
         settedValue.atPut(idx, true);
         baseFieldBuf.putLong(getOffset(idx), val.getTime());
         return true;
@@ -313,6 +316,9 @@ public boolean setDate(int idx, Date val) {
         if (!checkType(idx, Type.DataType.kDate)) {
             return false;
         }
+        if (val == null) {
+            return setNULL(idx);
+        }
         settedValue.atPut(idx, true);
         int dateVal = CodecUtil.dateToDateInt(val);
         baseFieldBuf.putInt(getOffset(idx), dateVal);
@@ -324,6 +330,9 @@ public boolean setString(int idx, String val) {
         if (!checkType(idx, Type.DataType.kString) && !checkType(idx, Type.DataType.kVarchar)) {
             return false;
         }
+        if (val == null) {
+            return setNULL(idx);
+        }
         if (settedValue.at(idx)) {
             return false;
         }

java/openmldb-common/src/test/java/com/_4paradigm/openmldb/common/RowCodecTest.java

@@ -89,6 +89,38 @@ public void testNull(String builderName) {
         }
     }
 
+    @Test(dataProvider = "builder")
+    public void testValueNull(String builderName) {
+        try {
+            List<ColumnDesc> schema = new ArrayList<ColumnDesc>();
+            schema.add(ColumnDesc.newBuilder().setName("col1").setDataType(DataType.kTimestamp).build());
+            schema.add(ColumnDesc.newBuilder().setName("col2").setDataType(DataType.kDate).build());
+            schema.add(ColumnDesc.newBuilder().setName("col3").setDataType(DataType.kVarchar).build());
+            RowBuilder builder;
+            if (builderName.equals("classic")) {
+                ClassicRowBuilder cBuilder = new ClassicRowBuilder(schema);
+                int size = cBuilder.calTotalLength(0);
+                ByteBuffer buffer = ByteBuffer.allocate(size).order(ByteOrder.LITTLE_ENDIAN);
+                cBuilder.setBuffer(buffer, size);
+                builder = cBuilder;
+            } else {
+                builder = new FlexibleRowBuilder(schema);
+            }
+            Assert.assertTrue(builder.appendTimestamp(null));
+            Assert.assertTrue(builder.appendDate(null));
+            Assert.assertTrue(builder.appendString(null));
+            Assert.assertTrue(builder.build());
+            ByteBuffer buffer = builder.getValue();
+            RowView rowView = new RowView(schema, buffer, buffer.capacity());
+            Assert.assertTrue(rowView.isNull(0));
+            Assert.assertTrue(rowView.isNull(1));
+            Assert.assertTrue(rowView.isNull(2));
+        } catch (Exception e) {
+            e.printStackTrace();
+            Assert.assertTrue(false);
+        }
+    }
+
     @Test(dataProvider = "builder")
     public void testNormal(String builderName) {
         try {