add testcase for issue #1613

Author: wenshao

core/src/test/java/com/alibaba/fastjson2/issues_1600/Issue1613.java

@@ -0,0 +1,19 @@
+package com.alibaba.fastjson2.issues_1600;
+
+import com.alibaba.fastjson2.JSON;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class Issue1613 {
+    @Test
+    public void test() {
+        Bean bean = new Bean();
+        assertEquals("{}", JSON.toJSONString(bean));
+    }
+
+    public static class Bean {
+        public void getId() {
+        }
+    }
+}