[Dubbo-3727] Fix NullPointerException when protocoluff protocol serializes an empty object

dubbo-serialization/dubbo-serialization-protostuff/src/main/java/org/apache/dubbo/common/serialize/protostuff/ProtostuffObjectOutput.java

@@ -47,7 +47,7 @@ public void writeObject(Object obj) throws IOException {
         byte[] classNameBytes;
 
         try {
-            if (WrapperUtils.needWrapper(obj)) {
+            if (obj == null || WrapperUtils.needWrapper(obj)) {
                 Schema<Wrapper> schema = RuntimeSchema.getSchema(Wrapper.class);
                 Wrapper wrapper = new Wrapper(obj);
                 bytes = GraphIOUtil.toByteArray(wrapper, schema, buffer);

dubbo-serialization/dubbo-serialization-test/src/test/java/org/apache/dubbo/common/serialize/protostuff/ProtostuffObjectOutputTest.java

@@ -0,0 +1,54 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.dubbo.common.serialize.protostuff;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.nullValue;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+public class ProtostuffObjectOutputTest {
+
+    private ByteArrayOutputStream byteArrayOutputStream;
+    private ProtostuffObjectOutput protostuffObjectOutput;
+    private ProtostuffObjectInput protostuffObjectInput;
+    private ByteArrayInputStream byteArrayInputStream;
+
+    @BeforeEach
+    public void setUp() throws Exception {
+        this.byteArrayOutputStream = new ByteArrayOutputStream();
+        this.protostuffObjectOutput = new ProtostuffObjectOutput(byteArrayOutputStream);
+    }
+
+    @Test
+    public void testWriteObjectNull() throws IOException, ClassNotFoundException {
+        this.protostuffObjectOutput.writeObject(null);
+        this.flushToInput();
+
+        assertThat(protostuffObjectInput.readObject(), nullValue());
+    }
+
+    private void flushToInput() throws IOException {
+        this.protostuffObjectOutput.flushBuffer();
+        this.byteArrayInputStream = new ByteArrayInputStream(byteArrayOutputStream.toByteArray());
+        this.protostuffObjectInput = new ProtostuffObjectInput(byteArrayInputStream);
+    }
+}