[ROCKETMQ-351] Add a default value to expressionType and polish the judgement. (#214)

common/src/main/java/org/apache/rocketmq/common/filter/ExpressionType.java

@@ -59,7 +59,7 @@ public class ExpressionType {
     public static final String TAG = "TAG";
 
     public static boolean isTagType(String type) {
-        if (type == null || TAG.equals(type)) {
+        if (type == null || "".equals(type) || TAG.equals(type)) {
             return true;
         }
         return false;

common/src/main/java/org/apache/rocketmq/common/protocol/heartbeat/SubscriptionData.java

@@ -21,6 +21,8 @@
 package org.apache.rocketmq.common.protocol.heartbeat;
 
 import com.alibaba.fastjson.annotation.JSONField;
+import org.apache.rocketmq.common.filter.ExpressionType;
+
 import java.util.HashSet;
 import java.util.Set;
 
@@ -32,7 +34,7 @@ public class SubscriptionData implements Comparable<SubscriptionData> {
     private Set<String> tagsSet = new HashSet<String>();
     private Set<Integer> codeSet = new HashSet<Integer>();
     private long subVersion = System.currentTimeMillis();
-    private String expressionType;
+    private String expressionType = ExpressionType.TAG;
 
     @JSONField(serialize = false)
     private String filterClassSource;