fix: allow order array to be empty

The "order" attribute can be empty in two cases:

- when setting "ordering: false"

Reference: https://datatables.net/reference/option/ordering
Related: #107

- with DataTabbles 2.0

> Columns can now have three ordering states - asc, desc or data index order (specified by an empty string). The columns.orderSequence option has been updated to make use of this, so the third ordering on a column will now take it back to the index order, rather than returning to the first direction.

Reference: https://cdn.datatables.net/2.0.0/
Related: #156

src/main/java/org/springframework/data/jpa/datatables/mapping/DataTablesInput.java

@@ -9,6 +9,11 @@
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 
+/**
+ * The format of the payload sent by the client.
+ *
+ * @see <a href="https://datatables.net/manual/server-side#Sent-parameters">datatables.net reference</a>
+ */
 @Data
 public class DataTablesInput {
   /**
@@ -55,7 +60,6 @@ public class DataTablesInput {
   /**
    * Order parameter
    */
-  @NotEmpty
   private List<Order> order = new ArrayList<>();
 
   /**