fix: 解决排序设置BUG

解决当设置 OrderBy.unsorted 后，调用 toSort 出现空异常

core/src/main/java/org/jfantasy/framework/dao/OrderBy.java

@@ -18,7 +18,7 @@
 public class OrderBy {
   private String property;
   private Direction direction;
-  private static final OrderBy UNSORTED = OrderBy.by(new OrderBy[0]);
+  private static final OrderBy UNSORTED = OrderBy.by();
   private final List<OrderBy> orders;
   private Sort.NullHandling nullHandling = Sort.NullHandling.NATIVE;
 
@@ -69,6 +69,10 @@ public static OrderBy by(List<OrderBy> orders) {
   }
 
   public Sort toSort() {
+    boolean isEmpty = direction == null && this.orders.isEmpty();
+    if (this == UNSORTED || isEmpty) {
+      return Sort.unsorted();
+    }
     if (this.isMulti()) {
       return Sort.by(
           this.getOrders().stream()
@@ -92,7 +96,7 @@ public String toString() {
     if (this.orders.isEmpty()) {
       return property + "_" + this.direction.name();
     }
-    return this.orders.stream().map(item -> item.toString()).collect(Collectors.joining(","));
+    return this.orders.stream().map(OrderBy::toString).collect(Collectors.joining(","));
   }
 
   public boolean isMulti() {

core/src/test/java/org/jfantasy/framework/dao/OrderByTest.java

@@ -0,0 +1,15 @@
+package org.jfantasy.framework.dao;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.data.domain.Sort;
+
+class OrderByTest {
+
+  @Test
+  void toSort() {
+    OrderBy orderBy = OrderBy.unsorted();
+    assertEquals(orderBy.toSort(), Sort.unsorted());
+  }
+}