[BUG]当对象的字段是复杂对象且值为空时，WriteNulls会导致NullPointerException

### 问题描述
* 当对象的字段是复杂对象且值为空时，WriteNulls会导致NullPointerException

### 环境信息
*请填写以下信息：*

- JDK信息： [e.g.：Openjdk 1.8.0_312]
- 版本信息：[e.g.：Fastjson2 2.0.59]

### 重现步骤

1. User对象下有Address对象字段
2. address字段值置为空
3. 调用JSONObject.from(user, JSONWriter.Feature.WriteNulls)
```java
import java.util.Date;
import java.util.LinkedHashMap;
import com.alibaba.fastjson2.JSONObject;
import com.alibaba.fastjson2.JSONWriter;

public class FastJson2Test {

public static void main(String[] args) {
User user = new User();
user.setId("U00001");
user.setRealName("Foo");
user.setCreateTime(new Date());
JSONObject json = JSONObject.from(user, JSONWriter.Feature.WriteNulls);
System.out.println(new LinkedHashMap<>(json));
}

@Data
protected static class User {
private String id;
private String realName;
private String remark;
private Address address;
private Date createTime;
}

@Data
protected static class Address {
private String id;
private String city;
}
}
```

### 相关日志输出
Exception in thread "main" java.lang.NullPointerException
at com.alibaba.fastjson2.writer.FieldWriterStringFunc.getFieldValue(FieldWriterStringFunc.java:36)
at com.alibaba.fastjson2.writer.ObjectWriterAdapter.toJSONObject(ObjectWriterAdapter.java:618)
at com.alibaba.fastjson2.writer.ObjectWriterAdapter.toJSONObject(ObjectWriterAdapter.java:610)
at com.alibaba.fastjson2.writer.ObjectWriterAdapter.toJSONObject(ObjectWriterAdapter.java:691)
at com.alibaba.fastjson2.JSON.toJSON(JSON.java:4027)
at com.alibaba.fastjson2.JSONObject.from(JSONObject.java:2259)
at FastJson2Test.main(FastJson2Test.java:14)

Reproduce the supplied User and Address example with JSONObject.from and JSONWriter.Feature.WriteNulls, then start at com.alibaba.fastjson2.writer.FieldWriterStringFunc.java:36 and the ObjectWriterAdapter.toJSONObject calls in the stack trace. Done means converting a User whose nested address is null completes without a NullPointerException and preserves the requested null-field behavior.

Written by the indexing model from the issue text.