add testcase for #3655

Author: wenshao

src/test/java/com/alibaba/json/bvt/issue_3600/Issue3655.java

@@ -0,0 +1,33 @@
+package com.alibaba.json.bvt.issue_3600;
+
+import com.alibaba.fastjson.JSON;
+import com.alibaba.fastjson.serializer.SerializerFeature;
+import junit.framework.TestCase;
+
+public class Issue3655 extends TestCase {
+
+    public void test1() {
+        B b = new B(null);
+        System.out.println(JSON.toJSONString(b, SerializerFeature.WriteNullStringAsEmpty));
+    }
+
+    public static abstract class A {
+        public abstract Object getData();
+    }
+
+    public static class B extends A {
+        private String data;
+
+        public B(String data) {
+            this.data = data;
+        }
+
+        public String getData() {
+            return this.data;
+        }
+
+        public void setData(String data) {
+            this.data = data;
+        }
+    }
+}