排序字段不对null进行限制，以解决dubbo报错的问题

search-api/src/main/java/com/uas/search/model/Sort.java

@@ -29,9 +29,6 @@ public class Sort implements Serializable {
 	 *            如果为true，则反转默认的升序
 	 */
 	public Sort(Field field, boolean reverse) {
-		if (field == null) {
-			throw new NullPointerException();
-		}
 		this.field = field;
 		this.reverse = reverse;
 	}
@@ -44,6 +41,10 @@ public class Sort implements Serializable {
 		this(field, false);
 	}
 
+	public Sort() {
+
+	}
+
 	/**
 	 * @return 排序的字段
 	 */
@@ -51,6 +52,10 @@ public class Sort implements Serializable {
 		return field;
 	}
 
+	public void setField(Field field) {
+		this.field = field;
+	}
+
 	/**
 	 * 排序是否应该被反转
 	 *