修复NullComparator反转无效问题

hutool-core/src/test/java/cn/hutool/core/comparator/PropertyComparatorTest.java

@@ -0,0 +1,55 @@
+package cn.hutool.core.comparator;
+
+import cn.hutool.core.collection.ListUtil;
+import lombok.AllArgsConstructor;
+import lombok.Data;
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class PropertyComparatorTest {
+	@Test
+	public void sortNullTest() {
+		final ArrayList<User> users = ListUtil.toList(
+				new User("1", "d"),
+				new User("2", null),
+				new User("3", "a")
+		);
+
+		// 默认null在末尾
+		final List<User> sortedList1 = ListUtil.sort(users, new PropertyComparator<>("b"));
+		Assert.assertEquals("a", sortedList1.get(0).getB());
+		Assert.assertEquals("d", sortedList1.get(1).getB());
+		Assert.assertNull(sortedList1.get(2).getB());
+
+		// null在首
+		final List<User> sortedList2 = ListUtil.sort(users, new PropertyComparator<>("b", false));
+		Assert.assertNull(sortedList2.get(0).getB());
+		Assert.assertEquals("a", sortedList2.get(1).getB());
+		Assert.assertEquals("d", sortedList2.get(2).getB());
+	}
+
+	@Test
+	public void reversedTest() {
+		final ArrayList<User> users = ListUtil.toList(
+				new User("1", "d"),
+				new User("2", null),
+				new User("3", "a")
+		);
+
+		// 反序
+		final List<User> sortedList = ListUtil.sort(users, new PropertyComparator<>("b").reversed());
+		Assert.assertNull(sortedList.get(0).getB());
+		Assert.assertEquals("d", sortedList.get(1).getB());
+		Assert.assertEquals("a", sortedList.get(2).getB());
+	}
+
+	@Data
+	@AllArgsConstructor
+	static class User{
+		private String a;
+		private String b;
+	}
+}