fix(json): 修复时间戳序列化时字段为空的问题

- 使用 ObjectUtil 判断字段名是否为空
- 增加对反射获取字段的空值检查
- 避免因字段为空导致的序列化异常

yudao-framework/yudao-common/src/main/java/cn/iocoder/yudao/framework/common/util/json/databind/TimestampLocalDateTimeSerializer.java

@@ -1,5 +1,6 @@
 package cn.iocoder.yudao.framework.common.util.json.databind;
 
+import cn.hutool.core.util.ObjectUtil;
 import cn.hutool.core.util.ReflectUtil;
 import com.fasterxml.jackson.annotation.JsonFormat;
 import com.fasterxml.jackson.core.JsonGenerator;
@@ -25,15 +26,17 @@ public class TimestampLocalDateTimeSerializer extends JsonSerializer<LocalDateTi
     public void serialize(LocalDateTime value, JsonGenerator gen, SerializerProvider serializers) throws IOException {
         // 情况一：有 JsonFormat 自定义注解，则使用它。https://github.com/YunaiV/ruoyi-vue-pro/pull/1019
         String fieldName = gen.getOutputContext().getCurrentName();
-        if (fieldName != null) {
+        if (ObjectUtil.isNotNull(fieldName)) {
             Class<?> clazz = gen.getOutputContext().getCurrentValue().getClass();
             Field field = ReflectUtil.getField(clazz, fieldName);
-            JsonFormat[] jsonFormats = field.getAnnotationsByType(JsonFormat.class);
-            if (jsonFormats.length > 0) {
-                String pattern = jsonFormats[0].pattern();
-                DateTimeFormatter formatter = DateTimeFormatter.ofPattern(pattern);
-                gen.writeString(formatter.format(value));
-                return;
+            if (ObjectUtil.isNotNull(field)) {
+                JsonFormat[] jsonFormats = field.getAnnotationsByType(JsonFormat.class);
+                if (jsonFormats.length > 0) {
+                    String pattern = jsonFormats[0].pattern();
+                    DateTimeFormatter formatter = DateTimeFormatter.ofPattern(pattern);
+                    gen.writeString(formatter.format(value));
+                    return;
+                }
             }
         }