add empty-[set] and empty-[set]-value attributes

Add two attributes in widget-mixin page :

• empty-[set]
• empty-[set]-value

'Available from version 0.17' is indicated for both of them.

source/import_documentation/Mixins/sorter-mixin.rst

@@ -53,7 +53,7 @@ Attributes
     The ``fields`` attribute of ``<solid-form-search>`` must at least contain ``field``. 
     ``Field`` attribute contains the fields of the resource to be offered to the user for sorting the data.
 
-    ``Order`` attribute allows you to propose 2 sorting orders: ascending (``asc`) and descending (``desc``). 
+    ``Order`` attribute allows you to propose 2 sorting orders: ascending (``asc``) and descending (``desc``). 
     By default, the order is ascending.
 
     The ``<solid-display>`` cannot combine ``order-asc`` or ``order-desc`` attribute AND ``sorted-by`` attribute.