[ISSUE #1463] Skip encode the null value property as string "null" (#1899)

* Update MessageDecoder.java
* add test case for MessageDecoder

common/src/main/java/org/apache/rocketmq/common/message/MessageDecoder.java

@@ -414,6 +414,9 @@ public class MessageDecoder {
                 final String name = entry.getKey();
                 final String value = entry.getValue();
 
+                if (value == null) {
+                    continue;
+                }
                 sb.append(name);
                 sb.append(NAME_VALUE_SEPARATOR);
                 sb.append(value);

common/src/test/java/org/apache/rocketmq/common/message/MessageDecoderTest.java

@@ -246,4 +246,23 @@ public class MessageDecoderTest {
 
         assertThat("abc").isEqualTo(decodedMsg.getTopic());
     }
-}
+
+    public void testNullValueProperty() throws Exception {
+        MessageExt msg = new MessageExt();
+        msg.setBody("x".getBytes());
+        msg.setTopic("x");
+        msg.setBornHost(new InetSocketAddress("127.0.0.1", 9000));
+        msg.setStoreHost(new InetSocketAddress("127.0.0.1", 9000));
+        String key = "NullValueKey";
+        msg.putProperty(key, null);
+        try {
+            byte[] encode = MessageDecoder.encode(msg, false);
+            MessageExt decode = MessageDecoder.decode(ByteBuffer.wrap(encode));
+            assertThat(decode.getProperty(key)).isNull();
+        } catch (Exception e) {
+            e.printStackTrace();
+            assertThat(Boolean.FALSE).isTrue();
+        }
+    }
+
+}
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