fix only return null when fist char is n, for issue #3537 (#3541)

* fix only return null when fist char is n, for issue #3537

* fix improve test case

core/src/main/java/com/alibaba/fastjson2/reader/FieldReaderObject.java

@@ -123,6 +123,7 @@ public class FieldReaderObject<T>
 
         Object value;
         try {
+            char first = jsonReader.current();
             if (jsonReader.nextIfNullOrEmptyString()) {
                 if (defaultValue != null) {
                     value = defaultValue;
@@ -135,7 +136,7 @@ public class FieldReaderObject<T>
                 } else if (fieldClass == Optional.class) {
                     value = Optional.empty();
                 } else {
-                    value = null;
+                    value = first == 'n' ? null : "";
                 }
             } else if (jsonReader.jsonb) {
                 if (fieldClass == Object.class) {

core/src/test/java/com/alibaba/fastjson2/issues_3500/Issue3537.java

@@ -0,0 +1,39 @@
+package com.alibaba.fastjson2.issues_3500;
+
+import com.alibaba.fastjson2.JSON;
+import org.junit.jupiter.api.Test;
+
+import java.util.Collections;
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+public class Issue3537 {
+    @Test
+    public void test() {
+        String json = "{\"value\":\"\"}";
+        SimpleTestValue s = JSON.parseObject(json, SimpleTestValue.class);
+        assertEquals("", s.getValue());
+        json = "{\"value\":null}";
+        s = JSON.parseObject(json, SimpleTestValue.class);
+        assertNull(s.getValue());
+    }
+
+    public static class SimpleTestValue {
+        private Object value;
+
+        // 标准 getter/setter
+        public Object getValue() {
+            return value;
+        }
+        public void setValue(Object value) {
+            this.value = value;
+        }
+
+        // 自定义 getter 会导致问题
+        public List<String> getRelates() {
+            return Collections.emptyList();
+        }
+    }
+}