修复XmlUtil.xmlToBean空节点转换失败问题

hutool-core/src/test/java/cn/hutool/core/util/Issue3136Test.java

@@ -19,8 +19,14 @@ import org.junit.Test;
 import java.util.ArrayList;
 import java.util.List;
 
+/**
+ * https://github.com/dromara/hutool/issues/3136
+ */
 public class Issue3136Test {
 
+	/**
+	 * 此用例中，message节点无content，理解为空节点，转换为map后，此节点值为""，转为对象时，理应为null
+	 */
 	@Test
 	public void xmlToBeanTest() {
 		final String xmlStr = "<?xml version=\"1.0\" encoding=\"gbk\" ?><response><code>02</code><message></message></response>";